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();