Reduce STT restart delay from 800ms to 300ms

Makes voice conversation feel much more responsive.
The 800ms delay was overkill - 300ms is sufficient for TTS audio to release.

app/(tabs)/_layout.tsx

@@ -207,7 +207,7 @@ export default function TabLayout() {
         if (sessionActiveRef.current) {
           safeStartSTT();
         }
-      }, 800); // 800ms to ensure TTS audio fully fades
+      }, 300); // 300ms to ensure TTS audio fully fades
       return () => clearTimeout(timer);
     }