Implement anonymous stats migration on signin

- Fix AuthModal to pass anonymousId on both signin and signup
- Add comprehensive migration logic in signin that moves anonymous completion stats to authenticated user
- Implement deduplication algorithm to handle overlapping completion dates
- Maintain earliest completion when duplicates exist

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/lib/components/AuthModal.svelte

@@ -95,7 +95,7 @@
 				method="POST"
 				action={mode === 'signin' ? '/auth/signin' : '/auth/signup'}
 				use:enhance={({ formData }) => {
-					if (mode === 'signup' && anonymousId) {
+					if (anonymousId) {
 						formData.append('anonymousId', anonymousId);
 					}
 					handleSubmit();

src/routes/auth/signin/+page.server.ts

@@ -1,12 +1,16 @@
 import { redirect, fail } from '@sveltejs/kit';
 import type { Actions } from './$types';
 import * as auth from '$lib/server/auth';
+import { db } from '$lib/server/db';
+import { dailyCompletions } from '$lib/server/db/schema';
+import { eq, inArray } from 'drizzle-orm';
 
 export const actions: Actions = {
 	default: async ({ request, cookies }) => {
 		const data = await request.formData();
 		const email = data.get('email')?.toString();
 		const password = data.get('password')?.toString();
+		const anonymousId = data.get('anonymousId')?.toString();
 
 		if (!email || !password) {
 			return fail(400, { error: 'Email and password are required' });
@@ -35,6 +39,63 @@ export const actions: Actions = {
 				return fail(400, { error: 'Invalid email or password' });
 			}
 
+			// Migrate anonymous stats if different anonymous ID
+			if (anonymousId && anonymousId !== user.id) {
+				try {
+					// Update all daily completions from the local anonymous ID to the user's ID
+					await db
+						.update(dailyCompletions)
+						.set({ anonymousId: user.id })
+						.where(eq(dailyCompletions.anonymousId, anonymousId));
+					
+					console.log(`Migrated stats from ${anonymousId} to ${user.id}`);
+
+					// Deduplicate any entries for the same date after migration
+					const allUserCompletions = await db
+						.select()
+						.from(dailyCompletions)
+						.where(eq(dailyCompletions.anonymousId, user.id));
+
+					// Group by date to find duplicates
+					const dateGroups = new Map<string, typeof allUserCompletions>();
+					for (const completion of allUserCompletions) {
+						const date = completion.date;
+						if (!dateGroups.has(date)) {
+							dateGroups.set(date, []);
+						}
+						dateGroups.get(date)!.push(completion);
+					}
+
+					// Process dates with duplicates
+					const duplicateIds: string[] = [];
+					for (const [date, completions] of dateGroups) {
+						if (completions.length > 1) {
+							// Sort by completedAt timestamp (earliest first)
+							completions.sort((a, b) => a.completedAt.getTime() - b.completedAt.getTime());
+							
+							// Keep the first (earliest), mark the rest for deletion
+							const toDelete = completions.slice(1);
+							duplicateIds.push(...toDelete.map(c => c.id));
+							
+							console.log(`Found ${completions.length} duplicates for date ${date}, keeping earliest, deleting ${toDelete.length}`);
+						}
+					}
+
+					// Delete duplicate entries
+					if (duplicateIds.length > 0) {
+						await db
+							.delete(dailyCompletions)
+							.where(inArray(dailyCompletions.id, duplicateIds));
+						
+						console.log(`Deleted ${duplicateIds.length} duplicate completion entries`);
+					}
+
+				} catch (error) {
+					console.error('Error migrating anonymous stats:', error);
+					// Don't fail the signin if stats migration fails
+				}
+			}
+
 			// Create session
 			const sessionToken = auth.generateSessionToken();
 			const session = await auth.createSession(sessionToken, user.id);