♻️ 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/Message/Agent/index.vue

@@ -11,7 +11,6 @@ defineProps<{
 </script>
 
 <template>
-    <!-- <pre class="max-w-full overflow-x-auto">{{ JSON.stringify(message, null, 2) }}</pre> -->
     <div class="flex flex-col w-full gap-2">
         <div v-for="part in message.parts" :key="part.id">
             <Reasoning v-if="part.type === 'reasoning'" :part="part" />
@@ -36,10 +35,10 @@ defineProps<{
 
         <div class="flex flex-row justify-between text-[var(--text-tertiary)] text-xs"
             v-if="message.generation && message.generation.status !== 'pending'">
-            <span class="flex items-center gap-1">
+            <div class="flex items-center gap-1">
                 <ModelIcon :size="12" :model-id="message.generation.modelId" />
                 {{ message.generation.modelId }}
-            </span>
+            </div>
 
             <div class="flex gap-2">
                 <span class="flex gap-1 items-center" v-if="message.generation.tokens?.output">