Adds src/main/themeMenuBuilder.js — a pure module that consumes
ThemeRegistry.list() + ThemeRegistry.categories() and the injected
setTheme/getCurrentThemeId callbacks to produce the View → Theme
submenu's MenuItemTemplate[] in the same shape as the previous
hardcoded block in src/main.js:1137-1245.
- Radio-style items with checked=true on the active theme id
- Grouped by category in registry order with separators between
non-empty categories
- No Electron / electron-store imports — keeps the module pure and
unit-testable under @jest-environment node
- Tests use jest.resetModules() + per-test require to avoid the
module-cache leakage the bootstrap test surfaced in T2
Amit Haridas
