♻️ 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/Markdown/Renderer.vue

@@ -41,11 +41,15 @@ const renderNode = (node: any, index: number): any => {
     return null;
 };
 
-defineRender(() => {
+const render = () => {
     const children = ast.value?.children?.flatMap(renderNode) || [];
 
     return h('div', { class: 'prose-wrapper' }, [
         h('article', { class: 'markdown-body' }, children)
     ]);
-})
+}
 </script>
+
+<template>
+    <render />
+</template>