Fix Ctrl-Q not quitting on Linux

[kevinsung]

Fixes #1023

Disclaimer: This PR and the following description were generated by Claude Code. I tested that the fix works on Linux, but I didn't test whether it changes behavior on other operating systems.

Fix Ctrl+Q not quitting the app on Linux

Summary

Ctrl+Q (the Quit accelerator) silently did nothing on Linux and Windows.

The Quit menu item used click: () => app.quit(). Menu items with a click
handler aren't run in the main process — buildMenu() in src/main.js rewrites
every click into an IPC message that's forwarded to the focused renderer,
which then re-runs its own copy of the handler. In the renderer, app (from
src/menu.js) is a stub object ({name, getVersion}) with no quit() method,
so the call threw/no-opped and the app never quit.

macOS was unaffected because the Quit item is spliced out of the File menu and
replaced with the native {role: 'quit'} there.

Fix

Route Quit through clickMain, the existing mechanism used by "New Window" for
actions that must run in the main process:

• src/menu.js: change the Quit item from click: () => app.quit() to
  clickMain: 'quit'.
• src/main.js: add quit: () => app.quit() to the clickMain dispatch map
  used by buildMenu().

With clickMain, the accelerator calls app.quit() directly in the main
process — no IPC roundtrip through the renderer's stub app.