Add History menu with Back/Forward keyboard shortcuts

[Copilot]

Adds Chrome-equivalent Back/Forward navigation shortcuts to the app menu bar under a new "History" menu, wired to the selected account's Gmail webContents navigation history.

Changes

• New "History" menu in packages/app/menu.ts, inserted between View and Accounts (matching Chrome's menu order)
  • Back: Cmd+[ (macOS) / Alt+Left (Windows/Linux)
  • Forward: Cmd+] (macOS) / Alt+Right (Windows/Linux)
• Each item calls navigationHistory.goBack() / navigationHistory.goForward() directly on the selected account's Gmail WebContentsView — the same Electron API backing the existing titlebar nav buttons
• Added scripts/*.js to .gitignore to prevent generated build artifacts from being tracked

Original prompt

implement keyboard shortcuts for back and forward. the shortcuts should be added to the app menu under a new menu "history" (like in chrome). the shortcuts should be same like in chrome (cmd + [ and cmd + ] and the equivalent for windows/linux). the shortcuts should trigger back and forward on the selected account gmail view.

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