Handles the Macroscope review on the Pi provider:

- startTurn resolves the prompt payload before installing the active turn,
  so an unreadable attachment can no longer wedge the thread with a turn
  that is never terminalized.
- Dialog responses and turn terminalization now share a semaphore, and the
  pending entry is dropped only after `extension_ui_response` is sent, so a
  failed send stays retryable and settlement cannot overtake resolution.
- Selecting "Pi default"/"inherit" again replays the baseline model and
  thinking level captured from the first `get_state`.
- Stop advertising the model-specific `xhigh`/`max` thinking levels, which
  made `set_thinking_level` fail and blocked the turn on models without them.
- Text generation rejects a model slug without a usable `provider/model`
  separator instead of silently running Pi's default model.
- Keep the raw RPC error payload as `cause` rather than in the message.
- Web Pi icon is a theme-aware glyph, matching its peers and the mobile icon.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>