refactor: reorganize Rust codebase into modular handlers and database layers

- Split monolithic src/db.rs (1122 lines) into domain modules: projects, tags, settings
- Extract API handlers from main.rs into separate handler modules by domain
- Add proxy module for ISR/SSR coordination with Bun process
- Introduce AppState for shared application context
- Add utility functions for asset serving and request classification
- Remove obsolete middleware/auth.rs in favor of session checks in handlers

web/src/app.css

@@ -170,7 +170,9 @@ html,
 body {
   @apply font-inter overflow-x-hidden;
   color: var(--color-text-primary);
-  transition: background-color 0.3s ease-in-out, color 0.3s ease-in-out;
+  transition:
+    background-color 0.3s ease-in-out,
+    color 0.3s ease-in-out;
 }
 
 body {