fix(terminal): restore copy and paste on Linux and Windows

[SametKUM]

Summary

Copy and paste never worked in Klustr's terminals on Linux and Windows — Exec, Node shell, the terminal drawer, and copying from the Logs viewers. Reported in discussion #90.

Root cause. On macOS the Wails Edit menu turns ⌘C / ⌘V into native clipboard commands and xterm.js leaves ⌘-chords alone, so it always worked there. Linux (WebKit2GTK) and Windows (WebView2) have no such menu; xterm.js maps Ctrl+C / Ctrl+V to the ^C / ^V control bytes and calls preventDefault(), so the webview never fires a copy / paste event and xterm's own clipboard handlers never run. Right-click was dead as well: release builds don't enable the webview's default context menu.

Fix.

• xtermClipboard.ts — attachCustomKeyEventHandler binds the terminal-convention chords on Linux/Windows: Ctrl+Shift+C / Ctrl+Insert copy, Ctrl+Shift+V / Shift+Insert paste, and a plain Ctrl+C copies while text is selected (SIGINT otherwise, matching Windows Terminal and VS Code). macOS is untouched — the matcher returns null there.
• Clipboard I/O goes through the Wails runtime (ClipboardGetText / ClipboardSetText) first: WebKitGTK has no navigator.clipboard.readText and WebView2 permission-gates it. Paste uses term.paste() so bracketed paste and newline normalisation survive.
• TerminalContextMenu.tsx — right-click Copy / Paste / Select all on every terminal surface (copy-only in the Logs viewers), focus handed back to xterm on close.
• Cheatsheet entries and guide docs.

Verification

• npm test (188 passing, 7 new for the chord matcher), npm run lint, npm run typecheck, npm run build.
• Manual on Linux (wails build -tags webkit2_41, WebKitGTK 2.52): Exec, Node shell, drawer and Logs — chords, selection-aware Ctrl+C followed by SIGINT, right-click menu, multi-line paste into bash and vim; no regressions on Ctrl+D/L/R, arrows, Tab.
• macOS: keyboard path unchanged by construction; the right-click menu is new.
• Windows: same code path (Wails disables WebView2 browser accelerator keys and implements the clipboard runtime); confirmation from a Windows build requested in Cut and paste to/from exec shell #90.

Refs #90

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