fix: graceful mic-unavailable handling over HTTP

navigator.mediaDevices.getUserMedia() requires a secure context
(HTTPS or localhost). When accessed over plain HTTP, the API
is undefined. Now shows a friendly chat message instead of a
cryptic TypeError in the console.

frontend/src/hooks/useConversation.ts

@@ -193,6 +193,12 @@ export function useConversation() {
   // ── Audio ──
 
   const startRecording = useCallback(async () => {
+    // Check if mediaDevices is available (requires HTTPS/localhost)
+    if (!navigator.mediaDevices || !navigator.mediaDevices.getUserMedia) {
+      addMessage('kira', 'Your browser needs HTTPS to use the microphone. Try accessing Kira through the HTTPS address instead!');
+      return;
+    }
+
     try {
       const stream = await navigator.mediaDevices.getUserMedia({ audio: true });
       streamRef.current = stream;