♻️ 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/ProviderSettings.vue

@@ -3,7 +3,7 @@ import { Providers } from '~/types/model';
 import { providerIcons } from '~/utils/model-mapping';
 
 const triplit = useTriplitClient();
-const { providers, unsubscribe: unsubscribeModels } = await useModels();
+const { providers } = useModels();
 
 if (providers.value === undefined) throw new Error('Providers not loaded');
 
@@ -37,15 +37,12 @@ const toggleProvider = async (id: string) => {
     });
 };
 
-onUnmounted(() => {
-    unsubscribeModels?.();
-});
-
 defineEmits(['navigate']);
 </script>
 
 <template>
-    <div class="flex flex-col gap-1">
+    <div
+        class="flex flex-col gap-1 py-4 overflow-auto [scrollbar-width:thin] [scrollbar-color:#888_transparent] [scrollbar-gutter:stable]">
         <h2 class="text-lg font-semibold flex items-center gap-2">
             Enabled <span class="text-sm bg-[var(--bg-container)] px-2 rounded-md py-0.5 text-[var(--text-secondary)]">
                 {{providers?.filter(p => p.enabled).length}}