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/lib/components/ThemeToggle.svelte

@@ -7,7 +7,9 @@
 <button
   type="button"
   onclick={() => themeStore.toggle()}
-  aria-label={themeStore.isDark ? "Switch to light mode" : "Switch to dark mode"}
+  aria-label={themeStore.isDark
+    ? "Switch to light mode"
+    : "Switch to dark mode"}
   class="relative size-9 rounded-md border border-zinc-300 dark:border-zinc-700 bg-zinc-100 dark:bg-zinc-900/50 hover:bg-zinc-200 dark:hover:bg-zinc-800/70 transition-all duration-200"
 >
   <div class="absolute inset-0 flex items-center justify-center">