♻️ 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/Sidenav/index.vue

@@ -109,7 +109,7 @@ const navKind = computed(() => {
             <div :style="{ minWidth: `${sidebarWidth}px` }" class="flex flex-col h-full justify-between">
                 <div class="flex flex-col h-full max-h-full overflow-y-hidden">
                     <!-- Header -->
-                    <div class="relative flex flex-row gap-2 justify-between items-center mb-1.5">
+                    <div class="relative flex flex-row gap-2 justify-between items-center mb-1.5 h-8">
 
                         <SidenavHeader v-if="navKind === 'home'" />
                         <SidenavHeaderAgent v-else-if="navKind === 'agent'" />
@@ -135,11 +135,8 @@ const navKind = computed(() => {
                     </div>
 
                     <!-- Main Menu -->
-                    <div
-                        class="max-h-full h-full overflow-auto [scrollbar-color:#888_transparent] [scrollbar-width:thin] [scrollbar-gutter:stable]">
-                        <SidenavNavHome v-if="navKind === 'home'" />
-                        <SidenavNavAgent v-else-if="navKind === 'agent'" />
-                    </div>
+                    <SidenavNavHome v-if="navKind === 'home'" />
+                    <SidenavNavAgent v-else-if="navKind === 'agent'" />
                 </div>
 
                 <div class="flex justify-between pt-2">