feat(flow): add Picture in Picture keep-alive mode

- Add FlowKeepAliveMode (Dynamic Island vs PiP) with iCloud sync
- Settings: keep-alive picker; inactivity timeout only for Live Activity
- PiP: waveform sample-buffer controller, mic released between utterances
- PiP sessions have no idle expiry; user closing PiP ends the session
- FlowSessionManager branches hostReady, session start, and utterance paths
- UIBackgroundModes picture-in-picture; bilingual strings and tests

Co-authored-by: Rocky <hkgood@users.noreply.github.com>

OSGKeyboardTests/AppGroupConfigurationTests.swift

@@ -33,6 +33,7 @@ final class AppGroupConfigurationTests: XCTestCase {
         XCTAssertEqual(config.polishIntensity, .default)
         XCTAssertTrue(config.personalDictionary.entries.isEmpty)
         XCTAssertTrue(config.flowSkipAppSwitch)
+        XCTAssertEqual(config.flowKeepAliveMode, .liveActivity)
         XCTAssertEqual(config.flowInactivityDuration, .fiveMinutes)
     }
 
@@ -58,6 +59,7 @@ final class AppGroupConfigurationTests: XCTestCase {
         config.cursorDragNavigationEnabled = false
         config.polishIntensity = .light
         config.flowSkipAppSwitch = false
+        config.flowKeepAliveMode = .pictureInPicture
         // Use a non-default value so the round-trip actually proves persistence.
         config.flowInactivityDuration = .threeHours
         config.save(to: defaults)
@@ -81,6 +83,7 @@ final class AppGroupConfigurationTests: XCTestCase {
         XCTAssertFalse(loaded.cursorDragNavigationEnabled)
         XCTAssertEqual(loaded.polishIntensity, .light)
         XCTAssertFalse(loaded.flowSkipAppSwitch)
+        XCTAssertEqual(loaded.flowKeepAliveMode, .pictureInPicture)
         XCTAssertEqual(loaded.flowInactivityDuration, .threeHours)
     }