refactor: consolidate utilities, clean up components, and improve theme persistence

- Consolidate common utilities (cn, format, time) into lib/utils
- Remove redundant utility files from pastel and units directories
- Clean up unused components (Separator, KeyboardShortcutsHelp)
- Relocate CommandPalette to components/units/ui/
- Force dark mode on landing page and improve theme persistence logic
- Add FOUC prevention script to RootLayout
- Fix sidebar height constraint in AppShell

lib/utils/index.ts

@@ -2,3 +2,5 @@ export * from './cn';
 export * from './debounce';
 export * from './urlSharing';
 export * from './animations';
+export * from './format';
+export * from './time';