Add Help menu with Documentation link and About dialog

[Copilot]

Adds a Help submenu to the native app menu bar (alongside File / Edit / View) with two items: a Documentation link that opens the project page in the system browser, and an About dialog with app metadata.

Changes

Rust (src-tauri/)

• New open_in_browser command — platform-specific URL opener (open / xdg-open / rundll32); rejects non-http(s) URLs to prevent misuse
• Permission entry (file-operations.toml) and capability grant (default.json) for the new command

TypeScript bridge

• tauriBridge.ts: openInBrowser(url) wrapping the new command

Help menu (useEditorController.ts)

• Builds and registers a Help submenu via @tauri-apps/api/menu:

  Help
  ├── Documentation          → opens github.com/MartinDejmal/mdedit in browser
  ├── ─────────────────────
  └── About mdedit           → opens in-app About modal
  

About dialog (src/features/ux/AboutDialog.tsx)

New modal component surfacing:

• App logo, name, and version (__APP_VERSION__)
• Author, license (GPL-3.0), and a clickable GitHub link
• Escape-to-close and focus management consistent with existing dialogs

Styles (styles.css)

Added .about-dialog, .about-header, .about-logo, .about-meta, and .about-link rules.