feat(terminal): on-screen Esc/Tab/Ctrl/arrow keys for phone keyboards (#117)

[Lexus2016]

Closes #117.

What

A row Esc · Tab · Ctrl · ← · ↓ · ↑ · → under the terminal pane, shown only on touch devices (@media (pointer: coarse)). Desktop layouts are unchanged.

• Each key sends the same {type:'input'} frame as a keystroke, through _reviveTerminal(), so a tap on a half-dead pane reconnects instead of being lost. Works on agent terminals and the engine pane.
• Arrows honour DECCKM: ESC [ A normally, ESC O A when the TUI switched on application cursor mode. Ctrl+arrow → ESC [1;5 X.
• Ctrl is one-shot: it applies to the next key, typed on Gboard (hooked into term.onData) or tapped in the row, then releases. aria-pressed shows the armed state.
• A tap never takes focus: the action runs on pointerdown with the default prevented, and touchstart is cancelled (non-passive). A focused button would close Gboard on every key.

Verification

• npm test → exit 0 (84 files). New test/terminal-keys.test.js (9 checks) runs the real block from index.html in a vm.
• Headless Chromium, Pixel 7 emulation (isMobile, hasTouch), real server, /ws/terminal mocked with page.routeWebSocket to record frames:
  • frames: \x1b, \t, \x1b[A, \x1b[D, Ctrl then typed c → \x03, Ctrl + ↓ → \x1b[1;5B, then typed x → x (Ctrl released)
  • focus before and after the taps: xterm-helper-textarea
  • row height 43 px, 0 page errors
• Desktop viewport (1280×800): row not visible.

Not done / risk

• Not tested on a physical Android phone with Gboard. Emulation covers touch events and the pointer media query, not Gboard's IME composition. If Gboard delivers a composed chunk instead of one character, the armed Ctrl applies to its first character only.
• No settings toggle: the row appears only on touch pointers, which covers the "only on mobile" ask without a new config key. A toggle can be added if someone needs it on a touch laptop.

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