♻️ 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/layouts/default.vue

@@ -24,10 +24,6 @@ useKeyboardShortcuts();
                     <Icon class="text-5" name="mynaui:panel-left-open" />
                 </button>
             </div>
-
-            <!-- <div class="flex items-center gap-2">
-                <div id="primary-loader-target"></div>
-            </div> -->
         </div>
         <div class="flex-1 overflow-y-auto">
             <slot />