Add right-click context menu for terminal panes

[PiratesIRC]

Summary

• Adds a DOM-based right-click context menu on terminal panes with Copy, Paste, Select All, Clear Terminal actions. Reuses the shared tab-context-menu CSS classes for consistent styling and adds a small shortcut-hint style.
• New module src/renderer/components/terminal-context-menu.ts exposes showTerminalContextMenu() / hideTerminalContextMenu(); integrated via a contextmenu listener on xtermWrap in terminal-pane.ts.
• Refactors a small pasteWithBracketedMode helper out of terminal-pane.ts into terminal-utils.ts so it can be reused by both the keyboard-paste path and the context menu Paste action.
• Co-located vitest coverage; CLAUDE.md updated.

Test plan

• Build passes (npm run build)
• All tests pass (npm test)
• Right-click in a terminal shows the menu at the cursor
• Copy works with an active selection (and is disabled/no-op when empty)
• Paste inserts clipboard contents using bracketed paste mode
• Select All highlights the full terminal buffer
• Clear Terminal wipes the buffer without killing the PTY
• Click outside / Escape dismisses the menu

Dependencies

Stacked on #115 (close-confirmation). The diff currently shows 23 commits because the branch is layered on top of #115's 15 commits — the actual feature is the last 8 commits (c99ef95 … a460a3c). Once #115 merges, I'll rebase this branch onto fresh main and force-push so the diff drops to a clean 8-commit / context-menu-only view. Reviewing only the terminal-context-menu commits is also a reasonable workflow.

🤖 Generated with Claude Code

[elirantutia]

Hey @PiratesIRC — could you rebase this onto current main and resolve the conflicts before I review? Note that #115 already shipped (commented over there), so the dependency chain doesn't apply — this should rebase straight onto main as a standalone PR. Thanks!

[PiratesIRC]

@elirantutia rebased onto current main and dropped the close-confirm commits — this is now standalone with 10 context-menu-only commits. Resolved conflicts in .gitignore, terminal-pane.ts (merged with the upstream WebGL/copy-on-select integration), and CLAUDE.md (kept upstream's board/team entries, added the terminal-context-menu component line). Build clean, 1525/1525 tests pass. Ready for review whenever you have time — thanks!