♻️ 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/ModelSelector.vue

@@ -192,8 +192,8 @@ useClickOutside(dropdownRef, closeDropdown);
 
 <template>
     <div ref="dropdownRef" class="relative">
-        <button @click="isOpen = !isOpen" ref="dropdownButton"
-            class="flex items-center gap-2 px-3 py-1.5 rounded-lg text-sm font-medium transition-colors duration-200"
+        <div role="button" @click="isOpen = !isOpen" ref="dropdownButton"
+            class="cursor-pointer flex items-center w-fit select-none gap-2 px-3 py-1.5 rounded-lg text-sm font-medium transition-colors duration-200"
             :class="[
                 isOpen
                     ? 'bg-[var(--color-hover)] text-[var(--text-primary)]'
@@ -206,7 +206,7 @@ useClickOutside(dropdownRef, closeDropdown);
             </span>
             <Icon name="mynaui:chevron-down" class="text-3.5 transition-transform duration-200"
                 :class="{ 'rotate-180': isOpen }" />
-        </button>
+        </div>
 
 
         <Transition enter-active-class="transition-all duration-150 ease-[cubic-bezier(0.5,_1,_0.89,_1)]"