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

app/globals.css

@@ -109,7 +109,7 @@
   }
 }
 
-:root {
+:root, .dark {
   /* CORPORATE DARK THEME (The Standard) */
   --background: #0a0a0f;
   --foreground: #ffffff;