refactor: extract recording code into separate modules with proper types

- Create extension/src/offscreen-types.ts with Chrome tabCapture constraint types
- Create extension/src/recording.ts extracting recording functions from background.ts
- Create playwriter/src/recording-relay.ts extracting recording logic from cdp-relay.ts
- Remove 'as any' usages by adding proper MediaStream constraint types
- Add typesafe message types for offscreen document communication
- Remove duplicate OUR_EXTENSION_IDS constant (uses EXTENSION_IDS from utils.ts)

extension/src/offscreen-types.ts

@@ -0,0 +1,106 @@
+/**
+ * Types for communication between background.ts and offscreen.ts.
+ * These are Chrome extension internal messages, separate from the WS protocol types.
+ */
+
+// Chrome-specific MediaStreamConstraints that TypeScript doesn't know about.
+// These use Chrome's proprietary tabCapture API constraints.
+// See: https://developer.chrome.com/docs/extensions/reference/tabCapture/
+export interface ChromeTabCaptureAudioConstraints {
+  mandatory: {
+    chromeMediaSource: 'tab'
+    chromeMediaSourceId: string
+  }
+}
+
+export interface ChromeTabCaptureVideoConstraints {
+  mandatory: {
+    chromeMediaSource: 'tab'
+    chromeMediaSourceId: string
+    minFrameRate?: number
+    maxFrameRate?: number
+  }
+}
+
+// Offscreen document message types
+export interface OffscreenStartRecordingMessage {
+  action: 'startRecording'
+  tabId: number
+  streamId: string
+  frameRate?: number
+  videoBitsPerSecond?: number
+  audioBitsPerSecond?: number
+  audio?: boolean
+}
+
+export interface OffscreenStopRecordingMessage {
+  action: 'stopRecording'
+  tabId: number
+}
+
+export interface OffscreenIsRecordingMessage {
+  action: 'isRecording'
+  tabId: number
+}
+
+export interface OffscreenCancelRecordingMessage {
+  action: 'cancelRecording'
+  tabId: number
+}
+
+export type OffscreenMessage =
+  | OffscreenStartRecordingMessage
+  | OffscreenStopRecordingMessage
+  | OffscreenIsRecordingMessage
+  | OffscreenCancelRecordingMessage
+
+// Offscreen document response types
+export type OffscreenStartRecordingResult = {
+  success: true
+  tabId: number
+  startedAt: number
+  mimeType: string
+} | {
+  success: false
+  error: string
+}
+
+export type OffscreenStopRecordingResult = {
+  success: true
+  tabId: number
+  duration: number
+} | {
+  success: false
+  error: string
+}
+
+export interface OffscreenIsRecordingResult {
+  isRecording: boolean
+  tabId: number
+  startedAt?: number
+}
+
+export type OffscreenCancelRecordingResult = {
+  success: true
+  tabId: number
+} | {
+  success: false
+  error: string
+}
+
+// Messages sent FROM offscreen TO background
+export interface OffscreenRecordingChunkMessage {
+  action: 'recordingChunk'
+  tabId: number
+  data?: number[] // Array from Uint8Array for message passing
+  final?: boolean
+}
+
+export interface OffscreenRecordingCancelledMessage {
+  action: 'recordingCancelled'
+  tabId: number
+}
+
+export type OffscreenOutgoingMessage =
+  | OffscreenRecordingChunkMessage
+  | OffscreenRecordingCancelledMessage