iPhone app: test pairing and power against a Holo Core stand-in

Valve publishes no Steam Frame OS image, so tests/frame-container builds the
Frame's SSH surface on Valve and Collabora's Holo Core aarch64 base: a
steamos user with a password and sudo, OpenSSH with keys and passwords,
Python, and a systemctl that only records requests.

Against it from the Simulator: password pairing, the host-key pin, the power
password check. Found and fixed: a changed host key or a refused login said
"Can't reach the Frame" and retried forever; they now say "Pair with the
Frame again" and offer that. The server's key rejection now says the header
may be wrong, not only missing.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ios/FrameControl/App/AppModel.swift

@@ -70,7 +70,9 @@ final class AppModel: ObservableObject {
             settings = target
         } catch {
             guard mine == attempt else { return }
-            fail((error as? FrameFailure)?.message ?? FrameLink.describe(error, host: target.host), retry: false)
+            let failure = error as? FrameFailure
+            fail(failure?.message ?? FrameLink.describe(error, host: target.host), retry: false,
+                 needsPairing: failure?.needsPairing ?? false)
             return
         }
         await connect()
@@ -197,11 +199,17 @@ final class AppModel: ObservableObject {
             forwarder?.stop()
             if let link { await link.close() }  // ends its server too
             guard current(), !(error is CancellationError) else { return }
-            fail((error as? FrameFailure)?.message ?? FrameLink.describe(error, host: settings.host), retry: true)
+            let failure = error as? FrameFailure
+            fail(failure?.message ?? FrameLink.describe(error, host: settings.host), retry: !(failure?.needsPairing ?? false),
+                 needsPairing: failure?.needsPairing ?? false)
         }
     }
 
-    private func fail(_ message: String, retry: Bool) {
+    /// Whether the last failure needs the user to pair again rather than wait.
+    @Published private(set) var needsPairing = false
+
+    private func fail(_ message: String, retry: Bool, needsPairing: Bool = false) {
+        self.needsPairing = needsPairing
         phase = .failed(message)
         retrying = retry && settings != nil
         guard retrying else { return }