feat: double-tap CTRL/ALT on the mobile key bar to lock it

[zarv1k]

Summary

The key bar's CTRL and ALT apply to the next character and then release. That cannot express a chord that repeats — most notably Claude Code's double Ctrl+C to exit, where the second ^C has to arrive within a second; re-arming CTRL between the two does not fit in that window.

A second tap within 500 ms of arming a modifier now locks it: the button gets a ring, and the modifier applies to every following character until the button is tapped again. A single tap behaves exactly as before.

• The logic lives in TerminalSession.tapModifier(), next to the existing pendingCtrl / pendingAlt state; handleInput() only clears a modifier that is not locked.
• The double-tap window only counts a tap that armed the modifier, so "CTRL, c, CTRL, d" typed quickly gives ^C ^D instead of locking by accident.
• README and the button titles mention the double tap.

Testing

• npm run typecheck and npm run build pass.
• Mounted the plugin in headless Chromium with touch emulation and a stub WebSocket, typing into xterm's textarea:
  • tap CTRL, c → ^C, then c → c (released);
  • double-tap CTRL, c c → ^C^C, still locked; tap CTRL, c → c (released);
  • tap CTRL, d, tap CTRL, d quickly → ^D^D, not locked;
  • two taps 700 ms apart → off, not locked;
  • double-tap ALT, b f → ESC b ESC f.
• Checked on an iPhone (installed PWA): double Ctrl+C exits Claude Code.
• npm test: resize reaches the PTY and a dropped connection keeps the shell alive… fail on unmodified main too in my environment (Debian 13, Node 24). This change only touches the frontend.

These three mobile PRs and #18 touch nearby code but merge together without conflicts in any order.