Fix the iPhone review's findings

- Cancelling (Change headset, Forget) invalidates the attempt in flight; a
  connection only becomes the app's once every step finished for it.
- A server that stops while the tunnel opens is noticed (exit callbacks are
  synchronised and replayed), and the app isn't left showing a dead page.
- The port is read only once the digits are complete, and range-checked.
- Other versions in ~/.cache/frame-control stay unless untouched for 14 days,
  so a second phone or iPad isn't cut off.
- The key is appended on its own line even if authorized_keys lacks a final
  newline.
- The app checks every 20 s, and on returning to the foreground, that the SSH
  session still answers, and reconnects if not.
- The ssh stand-in runs the command as its own process group and passes a
  TERM on to all of it, so a live-video ffmpeg stops with its stream.
- Touch screens show the library's Play buttons (the rule now follows the
  base one); the failure screen only says it's retrying when it is; the page
  says the app reconnects rather than naming a desktop menu.

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

ios/FrameControlTests/FrameControlTests.swift

@@ -29,6 +29,8 @@ final class HeadsetServerTests: XCTestCase {
     func testReadsThePortTheServerPrints() {
         XCTAssertEqual(HeadsetServer.port(in: "Frame Control on http://127.0.0.1:41234  (alias: frame; Ctrl-C to stop)\n"), 41234)
         XCTAssertNil(HeadsetServer.port(in: "Traceback (most recent call last):"))
+        XCTAssertNil(HeadsetServer.port(in: "Frame Control on http://127.0.0.1:4"))  // more digits may follow
+        XCTAssertNil(HeadsetServer.port(in: "Frame Control on http://127.0.0.1:99999 "))
     }
 
     func testBundleIsInTheApp() throws {