feat(mobile): add Shift arrows for Codex queued input and prompt navigation

[shenlvkang-collab]

Phone users cannot invoke Codex's Shift+Left (edit the last queued message) or Shift+Right (prompt stack back) from the keyboard accessory bar. Add ⇧← and ⇧→ beside Tab in both default and extended agent layouts, with accessible labels describing the Codex defaults (verified in Codex 0.154.0 /keymap).

The buttons send the standard modified-arrow sequences through sendNavKey, flushing any locally buffered draft before navigation and handing editing back to PTY echo. Taps refocus the terminal. The shell layout is unchanged.

Validation:

• Keyboard behavior suite: 65 tests passed, including draft flush order and the two escape sequences.
• Mobile keyboard runner: 11 relevant tests passed (accessory layout and shell keyboard regressions). Updated the layout assertion to include the new keys and the existing opt-in Read My Mind element.
• Playwright at 390px: clicked both buttons in both layouts using the real accessory module/styles in an isolated fixture; verified input request payloads and focus retention. This does not claim a physical-device or live Codex round-trip test.
• Typecheck, lint, frontend syntax, public asset checks, formatting, lockfile check, and production build passed.

Includes a patch changeset and README documentation. AI-assisted; implementation and validation reviewed locally.