Index: app/(app)/add/actions.ts
===================================================================
--- app/(app)/add/actions.ts	(revision 8563782f7b44a1553af5f70be2d12ab25a84d6bd)
+++ app/(app)/add/actions.ts	(revision 8563782f7b44a1553af5f70be2d12ab25a84d6bd)
@@ -0,0 +1,290 @@
+'use server';
+
+import { auth } from '@/auth';
+import { redirect } from 'next/navigation';
+import { sql } from '@/app/lib/db';
+import { revalidatePath } from 'next/cache';
+
+// ─── Shared helpers ────────────────────────────────────────────
+
+export type ActionState = {
+    error?: string;
+    success?: string;
+} | undefined;
+
+async function requireUserId(): Promise<number> {
+    const session = await auth();
+    if (!session?.user?.id) {
+        redirect('/login');
+    }
+    const userId = Number(session.user.id);
+    if (!Number.isInteger(userId)) {
+        redirect('/login');
+    }
+    return userId;
+}
+
+// ─── Add Transaction Account ───────────────────────────────────
+
+export async function addTransactionAccount(
+    _prev: ActionState,
+    formData: FormData,
+): Promise<ActionState> {
+    const userId = await requireUserId();
+
+    const name = String(formData.get('name') ?? '').trim();
+    const balanceRaw = String(formData.get('balance') ?? '0').trim();
+
+    if (!name) {
+        return { error: 'Account name is required.' };
+    }
+
+    const balance = Number(balanceRaw);
+    if (Number.isNaN(balance)) {
+        return { error: 'Balance must be a number.' };
+    }
+
+    try {
+        await sql`
+            INSERT INTO transaction_account (account_name, balance, user_id)
+            VALUES (${name}, ${balance}, ${userId})
+        `;
+    } catch {
+        return { error: 'Failed to create account.' };
+    }
+
+    revalidatePath('/dashboard');
+    revalidatePath('/add');
+    return { success: `Account "${name}" created.` };
+}
+
+// ─── Add Tag ───────────────────────────────────────────────────
+
+export async function addTag(
+    _prev: ActionState,
+    formData: FormData,
+): Promise<ActionState> {
+    await requireUserId(); // auth guard only
+
+    const name = String(formData.get('name') ?? '').trim().toLowerCase();
+
+    if (!name) {
+        return { error: 'Tag name is required.' };
+    }
+    if (name.startsWith('__note:')) {
+        return { error: 'Reserved tag prefix.' };
+    }
+
+    // Check duplicate (case-insensitive)
+    const existing = await sql`
+        SELECT tag_id FROM tag WHERE LOWER(tag_name) = ${name}
+    `;
+    if (existing.length > 0) {
+        return { error: `Tag "${name}" already exists.` };
+    }
+
+    try {
+        await sql`INSERT INTO tag (tag_name) VALUES (${name})`;
+    } catch {
+        return { error: 'Failed to create tag.' };
+    }
+
+    revalidatePath('/add');
+    return { success: `Tag "${name}" created.` };
+}
+
+// ─── Add Transaction ───────────────────────────────────────────
+
+type BreakdownInput = {
+    type: 'from' | 'to';
+    accountId: number;
+    amount: number;
+};
+
+export async function addTransaction(
+    _prev: ActionState,
+    formData: FormData,
+): Promise<ActionState> {
+    const userId = await requireUserId();
+
+    const name = String(formData.get('name') ?? '').trim();
+    const date = String(formData.get('date') ?? '').trim();
+    const amountRaw = String(formData.get('amount') ?? '').trim();
+    const tagsJson = String(formData.get('tags') ?? '[]');
+    const note = String(formData.get('note') ?? '').trim();
+    const breakdownsJson = String(formData.get('breakdowns') ?? '[]');
+
+    // ── Validation ──
+    if (!name) {
+        return { error: 'Transaction name is required.' };
+    }
+    if (!date) {
+        return { error: 'Date is required.' };
+    }
+
+    const amount = Number(amountRaw);
+    if (!amountRaw || Number.isNaN(amount)) {
+        return { error: 'Amount must be a number.' };
+    }
+
+    let tags: string[];
+    try {
+        tags = JSON.parse(tagsJson);
+        if (!Array.isArray(tags)) {
+            throw new Error();
+        }
+    } catch {
+        return { error: 'Invalid tags data.' };
+    }
+
+    let breakdowns: BreakdownInput[];
+    try {
+        breakdowns = JSON.parse(breakdownsJson);
+        if (!Array.isArray(breakdowns)) {
+            throw new Error();
+        }
+    } catch {
+        return { error: 'Invalid breakdowns data.' };
+    }
+
+    // Validate each breakdown
+    for (const breakdown of breakdowns) {
+        if (!['from', 'to'].includes(breakdown.type)) {
+            return { error: 'Invalid breakdown type.' };
+        }
+        if (!Number.isFinite(breakdown.amount) || breakdown.amount <= 0) {
+            return { error: 'Breakdown amounts must be positive.' };
+        }
+        if (!Number.isInteger(breakdown.accountId)) {
+            return { error: 'Invalid account in breakdown.' };
+        }
+    }
+
+    // Verify all accounts belong to this user
+    if (breakdowns.length > 0) {
+        const accountIds = [...new Set(breakdowns.map((b) => b.accountId))];
+        const ownedAccounts = await sql`
+            SELECT transaction_account_id
+            FROM transaction_account
+            WHERE user_id = ${userId}
+              AND transaction_account_id = ANY(${accountIds}::int[])
+        `;
+        if (ownedAccounts.length !== accountIds.length) {
+            return { error: 'One or more accounts do not belong to you.' };
+        }
+    }
+
+    // Compute net_amount from breakdowns
+    let totalEarned = 0;
+    let totalSpent = 0;
+    for (const breakdown of breakdowns) {
+        if (breakdown.type === 'to') {
+            totalEarned += breakdown.amount;
+        }
+        else {
+            totalSpent += breakdown.amount;
+        }
+    }
+    const netAmount = totalEarned - totalSpent;
+
+    try {
+        // Use a SQL transaction for atomicity
+        // eslint-disable-next-line @typescript-eslint/no-explicit-any
+        await sql.begin(async (tx: any) => {
+            // 1. Insert transaction
+            const [txRow] = await tx`
+                INSERT INTO transaction (transaction_name, amount, net_amount, date)
+                VALUES (${name}, ${amount}, ${netAmount}, ${date})
+                RETURNING transaction_id
+            `;
+            const transactionId: number = txRow.transaction_id;
+
+            // 2. Insert breakdowns & update account balances
+            for (const breakdown of breakdowns) {
+                const spent = breakdown.type === 'from' ? breakdown.amount : 0;
+                const earned = breakdown.type === 'to' ? breakdown.amount : 0;
+
+                await tx`
+                    INSERT INTO transaction_breakdown
+                        (transaction_id, transaction_account_id, spent_amount, earned_amount)
+                    VALUES (${transactionId}, ${breakdown.accountId}, ${spent}, ${earned})
+                `;
+
+                // Update balance: earned increases, spent decreases
+                await tx`
+                    UPDATE transaction_account
+                    SET balance = balance + ${earned} - ${spent}
+                    WHERE transaction_account_id = ${breakdown.accountId}
+                `;
+            }
+
+            // 3. Handle tags
+            for (const tagName of tags) {
+                const trimmed = tagName.trim().toLowerCase();
+                if (!trimmed || trimmed.startsWith('__note:')) {
+                    continue;
+                }
+
+                // Find or insert tag
+                const existing = await tx`
+                    SELECT tag_id FROM tag WHERE LOWER(tag_name) = ${trimmed}
+                `;
+                let tagId: number;
+                if (existing.length > 0) {
+                    tagId = existing[0].tag_id;
+                } else {
+                    const [inserted] = await tx`
+                        INSERT INTO tag (tag_name) VALUES (${trimmed})
+                        RETURNING tag_id
+                    `;
+                    tagId = inserted.tag_id;
+                }
+
+                // Check if assignment already exists
+                const existingAssignment = await tx`
+                    SELECT tag_assigned_to_transaction_id
+                    FROM tag_assigned_to_transaction
+                    WHERE transaction_id = ${transactionId} AND tag_id = ${tagId}
+                `;
+                if (existingAssignment.length === 0) {
+                    await tx`
+                        INSERT INTO tag_assigned_to_transaction (transaction_id, tag_id)
+                        VALUES (${transactionId}, ${tagId})
+                    `;
+                }
+            }
+
+            // 4. Handle note as a special __note: tag
+            if (note) {
+                const noteTagName = `__note:${note}`;
+
+                const existingNote = await tx`
+                    SELECT tag_id FROM tag WHERE tag_name = ${noteTagName}
+                `;
+                let noteTagId: number;
+                if (existingNote.length > 0) {
+                    noteTagId = existingNote[0].tag_id;
+                } else {
+                    const [inserted] = await tx`
+                        INSERT INTO tag (tag_name) VALUES (${noteTagName})
+                        RETURNING tag_id
+                    `;
+                    noteTagId = inserted.tag_id;
+                }
+
+                await tx`
+                    INSERT INTO tag_assigned_to_transaction (transaction_id, tag_id)
+                    VALUES (${transactionId}, ${noteTagId})
+                `;
+            }
+        });
+    } catch (e) {
+        console.error('addTransaction error:', e);
+        return { error: 'Failed to create transaction.' };
+    }
+
+    revalidatePath('/dashboard');
+    revalidatePath('/history');
+    revalidatePath('/add');
+    return { success: `Transaction "${name}" created.` };
+}