♻️ 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/Dialog.vue

@@ -6,7 +6,7 @@ import SystemAssistants from './SystemAssistants.vue';
 import AppearanceSettings from './AppearanceSettings.vue';
 import AIServiceProvider from './AIServiceProvider.vue';
 
-const { providers, unsubscribe: unsubscribeModels } = await useModels();
+const { providers } = useModels();
 
 const PAGES_CONFIG = {
     general: {
@@ -81,7 +81,6 @@ onUnmounted(() => {
     if (open.value) {
         document.body.removeEventListener('keydown', handleKeyDown);
     }
-    unsubscribeModels?.();
 });
 </script>
 
@@ -114,8 +113,8 @@ onUnmounted(() => {
                 <!-- DYNAMIC CONTENT -->
                 <main class="flex-1 flex flex-col overflow-hidden">
                     <div
-                        class="flex flex-col flex-1 p-3 bg-[var(--bg-surface)] overflow-y-auto border rounded-lg border-[var(--color-border)]">
-                        <header class="flex items-center justify-between pl-2 pb-2 ">
+                        class="max-h-full h-full flex flex-col flex-1 px-3 bg-[var(--bg-surface)] border rounded-lg border-[var(--color-border)]">
+                        <header class="flex items-center justify-between pl-2 pb-2 pt-2 ">
                             <h2 class="text-lg font-semibold m-0 capitalize">{{ runtimePage.label }}</h2>
                             <button
                                 class="hover:bg-[var(--color-hover)] p-1.5 rounded-md transition-colors duration-200 ease-[cubic-bezier(0,0.55,0.45,1)]"