♻️ 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/pages/agent/[id]/index.vue

@@ -8,16 +8,10 @@ const inputValue = ref('');
 const pendingMessage = ref<Message | null>(null);
 
 const { createTopic, sendMessage, autoRename } = useChat(route.params.id as string);
-const { getAgent, unsubscribe: unsubscribeAgents } = await useAgents();
-const { providers, unsubscribe: unsubscribeModels } = await useModels();
+const { getAgent } = useAgents();
+const { providers } = useModels();
 
-const agent = computed(() => {
-    if (route.params.id === null || typeof route.params.id !== 'string') {
-        throw new Error('Invalid agent ID');
-    }
-
-    return getAgent(route.params.id)!;
-});
+const agent = getAgent(route.params.id as string);
 
 if (!agent.value) navigateTo('/');
 
@@ -85,11 +79,6 @@ const handleSubmit = async (message: string, model: ModelWithProvider | null) =>
         }
     });
 };
-
-onUnmounted(() => {
-    unsubscribeAgents?.();
-    unsubscribeModels?.();
-});
 </script>