fix(android): reject stale shell backlog + always-visible keyboard toggle

[wahaj4311]

Summary

1. Terminal breakage on reconnect (`v1-Backend: terminal JSON-RPC stream closed`): the shared `remote_alleycat` read loop was accepting replayed backlog frames from a prior killed shell session while its own `shell_session_id` was still `None`. A stale `shell/exit` in the ring then broke the loop before `initialize` returned. Guard by treating an unset expected id as reject-until-spawned instead of accept-all.
2. Keyboard never appearing on Android: `onSingleTapConfirmed` returned false when `terminalRenderer` was null, so tapping the black surface during startup never called `showIme()`. Users had no other way to summon the IME. Fix by popping the IME on every confirmed tap, and add an `Icons.Outlined.Keyboard` `IconButton` in the terminal header wired to a new file-local `showTerminalKeyboard()` helper (via a `WeakReference` to the active `GhosttyAndroidSurfaceView`).

Pairs with dnakov/alleycat#PR (server-side session-replay fix for the shell agent). Either patch alone fixes the reconnect breakage; both together are the belt-and-suspenders.

Test plan

• Reject-on-null-session verified: Android app opens terminal, backgrounds, reconnects — every attach is Fresh with the server-side fix and no stale exit interrupts `initialize`.
• Manual: tap black viewport during startup — keyboard appears.
• Manual: tap the new keyboard icon in the terminal header — keyboard appears.
• Manual: reconnect after wifi flip — keyboard button still summons IME.