[ZoomIt] Theme-aware system-tray menu + a reusable helper for tray utilities

[crutkas]

Summary

Makes ZoomIt's system-tray context menu correctly follow the OS light/dark theme (including live theme switches), and factors the logic into a small reusable helper that any PowerToys system-tray utility can adopt.

Screenshots

ZoomIt tray context menu following the OS theme — Dark (top) and Light (bottom):

Why

Several PowerToys Win32 utilities have classic system-tray context menus, and the light/dark handling tends to be re-implemented per module (or missing). This PR fixes it for ZoomIt and introduces one shared, tested helper so other tray utilities can get the same behavior in a single call — and drop their own copies.

Reusable helper — src/common/Themes/dark_menu.h (header-only)

• SetAppMode(bool dark) — render this process's popup menus dark or light.
• IsSystemDarkMode() — current app theme from the documented AppsUseLightTheme value, read fresh.
• Drop-in:

  theme::dark_menu::SetAppMode(theme::dark_menu::IsSystemDarkMode());
  TrackPopupMenu(menu, ...);

• Header-only — no project/linker wiring; a module just #include <common/Themes/dark_menu.h>.

ZoomIt (first adopter)

• Tray menu now follows Light/Dark and updates live on a theme switch with no restart (runtime switches weren't picked up before).
• The OS draws the real themed menu — native keyboard, accessibility, checkmarks, separators and DPI; only the colors change.
• Standalone (non-PowerToys) build is unchanged (guarded by __ZOOMIT_POWERTOYS__).

Follow-up

Once this lands, a follow-up will adopt the helper in the other system-tray utilities (e.g. the runner tray menu) and remove their duplicate theme handling.

Testing

• MSBuild src\modules\ZoomIt\ZoomIt\ZoomIt.vcxproj /p:Configuration=Release /p:Platform=x64 /m — 0 warnings, 0 errors.
• Verified on Windows 11: tray menu in Light, in Dark, and a live Light↔Dark switch (menu follows without restart).