Desktop app: window title and platform menu basics

[regnull]

Goal

Give the desktop app a proper window title and an application menu suited to each platform. This is the item left over from #147.

Context

frontend/electron/main.cjs creates the BrowserWindow but never calls Menu.setApplicationMenu, so the app shows Electron's default menu, full of generic entries such as Help links to electronjs.org. The window title only comes from <title>OpenBot</title> in frontend/index.html, so it doesn't say which thread or page is open. In development (make app) the macOS app menu reads "Electron" instead of "OpenBot".

Acceptance criteria

• An explicit menu built from Electron roles: on macOS the app menu (About, Hide, Quit), Edit, View (reload and devtools only in development) and Window; on Linux the same without the app menu. Edit-menu shortcuts (copy/paste/undo) must keep working.
• Help links to the GitHub repository instead of Electron's site
• The window title shows the current page or thread (for example Thread name — OpenBot), updated by the renderer through document.title, with no preload changes
• The renderer security model is unchanged: no Node in the renderer, and preload stays minimal
• Unit tests for the menu template, matching the existing frontend/electron/*.test.ts