address CodeRabbit review: fix 4 critical + 12 major issues

Critical:
- Remove dead backend.utils.validation PyInstaller hidden import
- Fix story_items table rebuild to preserve track/trim/version columns
- Guard cache migration against same source/destination path
- Fix regeneration audio overwrite (use random uuid suffix per take)

Major:
- Engine selector: validate language on Qwen switch, clear stale modelSize
- Sync language validation regex between profile create and generate (22 langs)
- Guard CUDA download against duplicate concurrent requests
- Only set model_size for engines that support multiple sizes
- Fix 404 swallowed by generic except in history export
- Validate audio_path before FileResponse in export-audio
- Transcription: stream uploads in 1MB chunks, use robust cache check,
  call complete_download() on Whisper download success
- Set clean version as default when effects chain validation fails
- Return explicit error when Windows port occupied by non-voicebox process

tauri/src-tauri/src/main.rs

@@ -113,11 +113,17 @@ async fn start_server(
             &format!("127.0.0.1:{}", SERVER_PORT).parse().unwrap(),
             std::time::Duration::from_secs(1),
         ).is_ok() {
-            // Port is in use — check if it's a voicebox process via tasklist
+            // Port is in use — check if it's a voicebox process
             if let Some(pid) = find_voicebox_pid_on_port(SERVER_PORT) {
                 println!("Found existing voicebox-server on port {} (PID: {}), reusing it", SERVER_PORT, pid);
                 *state.server_pid.lock().unwrap() = Some(pid);
                 return Ok(format!("http://127.0.0.1:{}", SERVER_PORT));
+            } else {
+                return Err(format!(
+                    "Port {} is already in use by another application. \
+                     Close the other application or change the Voicebox port.",
+                    SERVER_PORT
+                ));
             }
         }
     }