♻️ 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/ProviderSidebar.vue

@@ -2,17 +2,13 @@
 import { providerIcons } from '~/utils/model-mapping';
 
 const { pageParams } = useSettings();
-const { providers, unsubscribe: unsubscribeModels } = await useModels();
-
-onUnmounted(() => {
-    unsubscribeModels?.();
-});
+const { providers } = await useModels();
 
 defineEmits(['navigate']);
 </script>
 
 <template>
-    <div class="flex flex-col gap-1">
+    <div class="flex flex-col gap-1 overflow-auto">
         <button @click="$emit('navigate', 'general')"
             class="flex items-center gap-2 p-2 rounded-lg text-sm hover:bg-[var(--color-hover)] transition-colors duration-200 ease-[cubic-bezier(0.5,_1,_0.89,_1)]">
             <Icon name="mynaui:chevron-left" class="text-4" /> Back to General