Create centralized theme management with useTheme composable

- Extract theme initialization logic from main.ts into useTheme composable
- Add setTheme() and initTheme() functions for consistent theme handling
- Update ThemePreferences to use useTheme instead of duplicate logic
- Remove unused DarkmodeToggle component
- Clean up main.ts from ~49 to 28 lines (theme logic now in composable)
- Establish single source of truth for theme management
- Follow Vue 3 composables pattern for better organization and testability

src/App.vue

@@ -13,8 +13,6 @@
     <!-- Popup that will show above existing rendered content -->
     <popup />
 
-    <darkmode-toggle />
-
     <!-- Command Palette -->
     <command-palette />
   </div>
@@ -25,7 +23,6 @@
   import NavigationHeader from "@/components/header/NavigationHeader.vue";
   import NavigationIcons from "@/components/header/NavigationIcons.vue";
   import Popup from "@/components/Popup.vue";
-  import DarkmodeToggle from "@/components/ui/DarkmodeToggle.vue";
   import CommandPalette from "@/components/ui/CommandPalette.vue";
 
   const router = useRouter();