fix(ios): adopt the UIScene life cycle so Xcode 27 builds launch

[chrisnojima]

Stack 1/9, splitting #29637. Base: master.

Why

iOS 27 requires an app to adopt the UIScene life cycle. Without it, a build made with Xcode 27 traps at launch inside _UIApplicationEvaluateRuntimeIssueForNoSceneLifecycleAdoption — the app does not start at all. That makes this a prerequisite for building the app with the current toolchain, independent of everything else in the stack.

What master does today

• No UIApplicationSceneManifest, no scene delegate: the app is still on the pre-scene UIApplication life cycle and cannot launch under Xcode 27.
• AppDelegate.didFinishLaunching builds a KeyboardWindow and starts React Native itself, so all root-view setup is tied to launch rather than to a scene connecting.
• Hardware key handling lives on that KeyboardWindow subclass.

What this changes and why each piece

• Info.plist gains a UIApplicationSceneManifest naming SceneDelegate; SceneDelegate subclasses Expo's ExpoAppSceneDelegate. This is the adoption itself.
• Root-view work moves out of didFinishLaunching into a new didStartReactNative(in:), because under scenes the window does not exist until a scene connects. BGTaskScheduler.register deliberately stays in didFinishLaunching — iOS requires it to run before that returns.
• KeyboardWindow is deleted and hardware enter / shift-enter move to pressesBegan on the delegate. Expo's scene delegate owns window creation now, so a custom UIWindow subclass has nowhere to be installed; the responder chain gives the same behaviour.
• The two RCTLinkingManager passthrough overrides are deleted because Expo's SceneEventForwarder already forwards them; keeping both would double-deliver. The keybase://incoming-share path re-adds an explicit call, since it does not go through the forwarder.
• applicationDidBecomeActive reports foreground directly. sceneDidBecomeActive is forwarded while UIApplication.applicationState still reads .inactive, so the old state-derived path would have concluded "not foreground" and stopped the local http server — which shows up as blank images.

Scope

No Go, JS or protocol change. The Swift symbol set is identical to master's, so no rebuilt Keybasego.xcframework is required. That is what lets this land first and alone.

Verification

iOS builds under Xcode 27 (BUILD SUCCEEDED, no warnings in our code).

Worth a manual pass, because these are behaviours the compiler cannot check: cold launch, images loading, hardware enter / shift-enter in the chat input, and a universal link plus a keybase:// link both cold and warm — the linking overrides were removed rather than relocated.

Ordering

Under scenes RN's AppState can stick at inactive (RCTAppState dedupes on last-known state), so mobileAppState stays wrong until #29651 takes the app state from the service. Merging this alone leaves that window open on master.