iOS: don't always prompt the user for camera/mic access on app startup (#1075)

* WIP: adding full app lifecycle event support, and Ctrl+C/signal catch

* iOS: don't always prompt the user for camera/mic access on app startup

This fixes two unrelated but similar-structure issues, both on iOS:

- `Cx::handle_repaint` ran a 32-slot encoder-capture sweep every frame,
  which lazy-created `AvCaptureAccess` and fired
  `requestAccessForMediaType:AVMediaTypeVideo` — i.e. the camera prompt —
  on every Makepad app, even ones that never touch the camera. Gate the
  loop on `av_capture.is_some()` and stop the three `video_encoder_*`
  methods in `apple_media.rs` from lazy-creating it; they now return
  `EncoderNotStarted` (or no-op for `push_frame`) when no encoder exists.

- `AudioUnitAccess::new()` unconditionally activated the shared
  `AVAudioSession` with category `PlayAndRecord` + `VoiceChat` mode,
  which is wrong for playback-only apps (mic prompt + forced VPIO).
  Defer session config to a new `ensure_ios_session(for_input)` helper,
  called from `use_audio_inputs` (PlayAndRecord) and `use_audio_outputs`
  (Playback). Idempotent, never downgrades.

* Added fuller lifecycle event support, plus ability to catch sigquit

Tested working well on every platform except web/wasm, as I don't have
a working setup able to test that.