fix(frontend): wire clipboard copy/paste in XtermTerminal

[neversettle17-101]

Summary

• XtermTerminal loaded several xterm.js addons but had no clipboard wiring, so neither the OS Copy command nor plain mouse drag-to-select worked against the zellij-attached PTY (zellij owns SGR mouse tracking, and xterm selection is rendered to canvas, not DOM).
• Added an attachCustomKeyEventHandler that intercepts Cmd/Ctrl+C (copies term.getSelection() via navigator.clipboard.writeText only when there's an active selection, otherwise lets the event through so Ctrl+C still sends SIGINT) and Cmd/Ctrl+V (reads navigator.clipboard.readText() and forwards via term.paste, rather than relying on xterm's native DOM paste event).
• Shift+drag for local text selection while zellij owns mouse tracking is xterm's existing default behavior — no change needed there.
• No new dependency: implemented with the existing @xterm/xterm APIs, no @xterm/addon-clipboard needed.

Test plan

• pnpm -C frontend typecheck passes
• pnpm -C frontend test (full renderer suite, including XtermTerminal tests) passes — 140/140
• Manual: open a session terminal, Shift+drag to select text, Cmd/Ctrl+C to copy, paste into another app
• Manual: Cmd/Ctrl+V into the terminal pastes clipboard text into the PTY
• Manual: Ctrl+C with no selection still sends SIGINT to the running process
• Manual smoke test on Linux/Windows (no native Electron menu there, so this relies entirely on the explicit key handling added here)

Fixes #305

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