fix: address codebase review issues

- refactor(backend): use tokio::sync::Mutex for DbState and update commands to async
- fix(backend): remove unconditional post-queue system action in scheduler
- refactor(backend): remove dead WebSocket aria2 progress loop
- fix(backend): use character count for deep link payload length check
- fix(backend): implement dynamic port fallback for extension server
- build(backend): apply macos codesigning step for release builds
- security(backend): add explicitly defined Content-Security-Policy
- fix(frontend): replace pause_download API call with remove_download for file cleanup
- fix(frontend): resolve bug ignoring 0% progress reporting
- fix(frontend): append instead of overwrite deep link URLs when Add Modal is open
- style(frontend): append standard .dark class for dark mode themes
- style(frontend): remove ghost row layout hack from download table
- build: decouple typescript binding generation from build step

src/App.tsx

@@ -162,8 +162,12 @@ function App() {
       if (theme === 'system') {
         const systemDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
         root.classList.add(systemDark ? 'theme-dark' : 'theme-light');
+        if (systemDark) root.classList.add('dark');
       } else {
         root.classList.add(`theme-${theme}`);
+        if (['dark', 'dracula', 'nord'].includes(theme)) {
+          root.classList.add('dark');
+        }
       }
     };