feat: Add comprehensive backend validation, toast notifications, and Tokyo Night theme

- Backend: Add server-side validation with go-playground/validator/v10
- Frontend: Add toast notifications for API errors with @zerodevx/svelte-toast
- UI: Complete Tokyo Night theme redesign with modern animations
- Docs: Update COMPLETE_DOCUMENTATION.md and README.md with all enhancements
- Validation: Email format, password strength, and input sanitization
- UX: Real-time error feedback, loading states, and responsive design

backend/sqlc.yaml

@@ -0,0 +1,17 @@
+version: "2"
+sql:
+  - engine: "postgresql"
+    schema: "migrations"
+    queries: "internal/database/queries"
+    gen:
+      go:
+        package: "database"
+        out: "internal/database"
+        sql_package: "pgx/v5"
+        emit_db_tags: true
+        emit_prepared_queries: true
+        emit_interface: true
+        emit_exact_table_names: true
+        emit_empty_slices: true
+        emit_exported_queries: true
+        emit_json_tags: true