fix: mobile menu overlay, speech bubble timing, TTS static file fallback

- Mobile nav menu now overlays content (absolute positioning) instead of
  pushing it down
- Speech bubbles stay visible for minimum 400ms even when TTS resolves
  instantly or fails
- kokoroPlayCached checks audio cache and loads static files even when
  Kokoro worker hasn't loaded — fixes TTS not playing on production
- CORS_ORIGIN env now supports comma-separated origins
- Rename "VIDEO REPLAY" to play icon + "REPLAY"

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/NavBar.vue

@@ -85,7 +85,7 @@ const bottomNav = [
       </button>
     </div>
 
-    <div v-if="isMenuOpen" class="md:hidden border-t border-border px-6 py-4 space-y-4 bg-surface">
+    <div v-if="isMenuOpen" class="md:hidden absolute left-0 right-0 top-full border-t border-border px-6 py-4 space-y-4 bg-surface/95 backdrop-blur-md z-50">
       <RouterLink
         v-for="link in links"
         :key="link.to"