feat: Improve UI layout and navigation

- Increase logo size (48x48 desktop, 56x56 mobile) for better visibility
- Add logo as favicon
- Add logo to mobile header
- Move user menu to navigation bars (sidebar on desktop, bottom bar on mobile)
- Fix desktop chat layout - container structure prevents voice controls cutoff
- Fix mobile bottom bar - use icon-only ActionIcons instead of truncated text buttons
- Hide Create Node/New Conversation buttons on mobile to save header space
- Make fixed header and voice controls work properly with containers

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lib/ai.ts

@@ -1,17 +1,25 @@
 import { GoogleGenerativeAI } from '@google/generative-ai';
 
-const genAI = new GoogleGenerativeAI(process.env.GOOGLE_API_KEY!);
+// Validate required environment variables
+if (!process.env.GOOGLE_GENERATIVE_AI_API_KEY) {
+  throw new Error('GOOGLE_GENERATIVE_AI_API_KEY environment variable is required');
+}
+
+if (!process.env.GOOGLE_EMBEDDING_MODEL) {
+  throw new Error('GOOGLE_EMBEDDING_MODEL environment variable is required (e.g., gemini-embedding-001)');
+}
+
+const genAI = new GoogleGenerativeAI(process.env.GOOGLE_GENERATIVE_AI_API_KEY);
 
 const embeddingModel = genAI.getGenerativeModel({
-  model: 'text-embedding-004',
+  model: process.env.GOOGLE_EMBEDDING_MODEL,
 });
 
 /**
- * Generates a vector embedding for a given text using Google's text-embedding-004 model.
- * The output is a 768-dimension vector (not 1536 as originally specified).
+ * Generates a vector embedding for a given text using the configured Google embedding model.
  *
  * @param text - The text to embed
- * @returns A 768-dimension vector (Array<number>)
+ * @returns A vector embedding (dimension depends on model)
  */
 export async function generateEmbedding(text: string): Promise<number[]> {
   try {