Add native right-click context menus (copy/paste, spellcheck, links, images)

## Problem

Right-click does almost nothing in the app today. Electron doesn't provide context menus by default — apps must wire them up themselves, and we currently don't. Concretely:

- No Cut/Copy/Paste menu in text inputs, including the chat composer
- No "Copy" on selected text in emails, chat messages, or notes
- No spell-check suggestions (Chromium's spellchecker is on, but suggestions only surface via a context menu)
- No "Copy Link Address" on links, no "Save Image" on images
- No context menu in the embedded browser pane

The only right-click menus that exist are custom in-app (Radix UI) menus on a few list rows — chat history entries, notes, knowledge items, workspace files — plus the tray icon menu. Those should keep working as-is.

## Suggested approach

Use the [`electron-context-menu`](https://github.com/sindresorhus/electron-context-menu) package — it provides editing, spellcheck, link, and image menus out of the box:

1. `cd apps/x/apps/main && pnpm add electron-context-menu`
2. Initialize it in the main process (`apps/x/apps/main/src/main.ts`); esbuild bundles the dependency automatically
3. Make sure it also attaches to the browser pane's `WebContentsView`, not just the main window
4. Check it doesn't fight the existing custom Radix menus — the native menu naturally only shows for editable fields, selected text, links, and images, which shouldn't overlap with the list-row menus

## Acceptance criteria

- [ ] Right-click in a text input shows Cut/Copy/Paste (and spellcheck suggestions on a misspelled word)
- [ ] Right-click on selected text anywhere shows Copy
- [ ] Right-click on a link/image in the browser pane offers Copy Link / Save Image
- [ ] Existing custom right-click menus (chat history, notes, etc.) still work

Dev setup is in `CLAUDE.md` / `README` — `cd apps/x && pnpm install && npm run dev`.