refactor: simplify screen recording code

- Fix vite output path (was broken lib/background.mjs, now background.js)
- Import recording types from protocol.ts instead of duplicating in background.ts
- Remove dead getSessionId() and unused sessionId params from screen-recording.ts
- Replace 4 executor wrapper functions with single withRecordingDefaults helper
- Fix binary chunk routing with lastRecordingMetadataTabId tracking
- Add ExtensionStopRecordingResult type for proper extension/relay separation

Saves ~56 lines while improving correctness of multi-tab recording.

extension/vite.config.mts

@@ -58,12 +58,7 @@ export default defineConfig({
         welcome: resolve(__dirname, 'src/welcome.html'),
       },
       output: {
-        entryFileNames: (chunkInfo) => {
-          if (chunkInfo.name === 'background') {
-            return 'lib/background.mjs';
-          }
-          return '[name].js';
-        },
+        entryFileNames: '[name].js',
         format: 'es',
       },
     },