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

components/providers/ThemeProvider.tsx

@@ -15,16 +15,21 @@ const ThemeContext = createContext<ThemeContextType | undefined>(undefined);
 export function ThemeProvider({ children }: { children: React.ReactNode }) {
   const [theme, setTheme] = useState<Theme>('dark');
   const [resolvedTheme, setResolvedTheme] = useState<'light' | 'dark'>('dark');
+  const [mounted, setMounted] = useState(false);
 
+  // Load theme from localStorage on mount
   useEffect(() => {
-    // Load theme from localStorage
     const stored = localStorage.getItem('theme') as Theme | null;
     if (stored) {
       setTheme(stored);
     }
+    setMounted(true);
   }, []);
 
+  // Apply theme to document element and save to localStorage
   useEffect(() => {
+    if (!mounted) return;
+
     const root = window.document.documentElement;
 
     // Remove previous theme classes
@@ -43,10 +48,12 @@ export function ThemeProvider({ children }: { children: React.ReactNode }) {
 
     // Save to localStorage
     localStorage.setItem('theme', theme);
-  }, [theme]);
+  }, [theme, mounted]);
 
   // Listen for system theme changes
   useEffect(() => {
+    if (!mounted) return;
+
     const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
 
     const handleChange = () => {
@@ -60,7 +67,7 @@ export function ThemeProvider({ children }: { children: React.ReactNode }) {
 
     mediaQuery.addEventListener('change', handleChange);
     return () => mediaQuery.removeEventListener('change', handleChange);
-  }, [theme]);
+  }, [theme, mounted]);
 
   return (
     <ThemeContext.Provider value={{ theme, setTheme, resolvedTheme }}>