WIP: screen recording via chrome.tabCapture

Work in progress for navigation-safe screen recording.

Implemented:
- tabCapture permission in manifest
- Recording handlers in extension background.ts
- Recording endpoints in relay server
- Binary WebSocket streaming for video chunks
- Protocol types for recording messages
- Executor integration

Needs:
- Offscreen document implementation (tabCapture not available in service workers)
- Connect background.ts to offscreen document via messages
- Update manifest for offscreen document

The offscreen.ts/html files are scaffolding for this next step.

extension/src/types.ts

@@ -8,6 +8,7 @@ export interface TabInfo {
   errorText?: string
   pinnedCount?: number
   attachOrder?: number
+  isRecording?: boolean
 }
 
 export interface ExtensionState {
@@ -16,3 +17,15 @@ export interface ExtensionState {
   currentTabId: number | undefined
   errorText: string | undefined
 }
+
+/** 
+ * Recording state - kept separate from store since MediaRecorder/MediaStream can't be serialized.
+ * Note: MediaRecorder and MediaStream types are available in the extension's browser context.
+ */
+export interface RecordingInfo {
+  tabId: number
+  startedAt: number
+  recorder: any // MediaRecorder - using any to avoid DOM type dependency
+  stream: any // MediaStream - using any to avoid DOM type dependency
+  mimeType: string
+}