Add right-click context menu in the editor buffer (#315)

[sinelaw]

Summary

Closes #315.

Right-clicking inside a text-buffer split previously did nothing — the mouse handler only raised context menus for tabs, the file explorer, and the orchestrator dock. The requester asked for a VS Code-style right-click menu in the editor (without Shift, which today just triggers terminal paste passthrough).

This adds an editor context menu offering the universal clipboard verbs:

┌──────────────────┐
│ Cut              │
│ Copy             │
│ Paste            │
│ Select All       │
└──────────────────┘

It deliberately stays minimal — the command palette (Ctrl+P) already covers the long tail of commands; this is just the muscle-memory right-click surface.

Design

• Mirrors the existing FileExplorerContextMenu / TabContextMenu machinery for state, rendering, keyboard navigation (Up/Down/Enter/Esc), mouse hover-highlight, and click-to-activate. New EditorContextMenu type in app/types/context_menu.rs.
• Each item dispatches the corresponding editor Action (Cut/Copy/Paste/SelectAll), so all existing routing is reused unchanged — read-only buffers, focused text-widget inputs, composite buffers, the internal/system clipboard split, etc.
• Right-clicking a split focuses it first, so the clipboard action targets the buffer the user clicked.
• Terminal splits are left untouched so they keep their own mouse semantics (e.g. Shift+Right-Click paste passthrough). The menu only opens over non-terminal buffer splits.
• Typing any normal key dismisses the menu and falls through to normal input handling (the keystroke isn't swallowed); Esc / click-outside also dismiss.

Testing

• New e2e test tests/e2e/editor_context_menu.rs (12 cases) drives real mouse/keyboard events through the same handle_mouse/handle_key path as production and asserts only on rendered output (per the "observe, not inspect" rule): menu open/close, item rendering, Cut/Copy→Paste/Select All behavior, keyboard nav, hover, terminal/menu-bar exclusion.
• Manually validated in a tmux session against the real fresh binary: a right-click renders the menu and clicking Select All highlights the whole buffer and moves the cursor to EOF.
• cargo check, cargo clippy --all-targets, and cargo fmt are clean for the changed files.
• Adds editor.context.* locale keys across all 14 locales.

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