`Menu.setApplicationMenu(null)` drops the entire default Electron
menu so students don't see a distracting File/Edit/View bar — but it
also drops the default keyboard accelerators that came with that
menu, including the DevTools toggle. Discovered this when a grey-
screen failure (#3.0.0/3.0.1) left a user with no way to inspect the
error because Ctrl+Shift+I did nothing.

Hooked `before-input-event` directly on the main window to re-attach
three accelerators without restoring the menu bar:
- Ctrl+Shift+I  (Cmd+Shift+I on mac) → toggle DevTools
- F12 → toggle DevTools (Windows muscle memory)
- Ctrl+R → reload renderer

Pairs with the safety net in v3.0.2 — even if the renderer paints
the error to the DOM directly, the user can now also pop DevTools to
inspect network / console / runtime state.

v3.0.3

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>