♻️ refactor: optimize state management, switch to @tanstack/vue-virtual, and improve performance

- Centralize `useAgents` and `useModels` state within the Nuxt app context to prevent data leaks and improve initialization.
- Migrate virtualization from `vue-virtual-scroller` to `@tanstack/vue-virtual` with new `RowVirtualizerFixed` and `RowVirtualizerDynamic` components.
- Upgrade Nuxt to v4.3.1 and remove `@vue-macros/nuxt`.
- Replace `big.js` with an optimized custom `lshDecimal` string manipulation logic for pricing calculations in the provider API.
- Implement automatic focus redirection in `ChatInput` to capture standard keyboard input.
- Refactor Sidenav and Settings components to utilize virtualization for long lists (topics, agents, models).
- Enhance theme colors and mobile experience. More work to come on both of these.

app/components/Settings/AppearanceSettings.vue

@@ -22,7 +22,8 @@ defineEmits(['navigate']);
 </script>
 
 <template>
-    <div class="flex flex-col gap-6 mt-4">
+    <div
+        class="flex flex-col gap-6 py-4 overflow-auto [scrollbar-width:thin] [scrollbar-color:#888_transparent] [scrollbar-gutter:stable]">
         <div class="flex flex-row items-center justify-between gap-2">
             <h4 class="font-medium">Theme</h4>
             <div class="flex gap-2">