Fix Windows CI build and start-page theme colors (Qt 6.7.3)

[Nicolas01]

Problem

The Windows CI build (build win64) had been failing on master for a long stretch, while Linux and macOS passed. The failure was a Qt header error during compilation of src/cli/Installer.cpp:

qvarlengtharray.h(963): error C3861: 'stdext': identifier not found
qvarlengtharray.h(963): error C2065: 'stdext': undeclared identifier

Root cause

The CI matrix pinned Qt 6.6.0 but runs on windows-latest, which GitHub rolled forward to the VS2026 / Windows Server 2025 image. Recent MSVC STL versions removed the Microsoft stdext::* array-iterator extension that Qt 6.6.0's qvarlengtharray.h still references under Q_CC_MSVC. Only MSVC enters that code path, which is why Linux (GCC) and macOS (clang) were unaffected. This is environment bit-rot, not a code regression.

Fix

Bump the CI Qt version 6.6.0 -> 6.7.3 (the stdext usage was removed from Qt in 6.6.2). Validating that bump on a fork surfaced several further incompatibilities with Qt 6.7 and the current runner images, each fixed here:

1. Windows stdext build error — bump Qt to 6.7.3.
2. Linux Qt install — aqt renamed the desktop arch in 6.7; gcc_64 -> linux_gcc_64.
3. Windows Configure failure — QWindowsVistaStylePlugin was renamed to QModernWindowsStylePlugin in Qt 6.7. Also made the pack/ plugin loop skip a missing target on all platforms (previously it only continue()d on UNIX/APPLE, so a missing target hard-failed configure on Windows).
4. Linux Configure failure — Qt6PrintSupport now needs CUPS on Ubuntu 24.04; install libcups2-dev.
5. Linux AppImage — aqt downloads the linux_gcc_64 package but extracts it to gcc_64, so the hand-built QTDIR pointed at a nonexistent dir; use the QT_ROOT_DIR exported by install-qt-action.
6. publish job — gh release failed with not a git repository because the job has no checkout; pass --repo ${{ github.repository }}.

Windows theme regression fix

With the Windows build restored, the produced app showed unreadable white-on-white text on the start page (the "theme bug" that previously led to the Windows .exe being dropped from releases).

Root cause: on Windows, Application sets an explicit base palette (setPalette(style()->standardPalette())) before installing the theme style. Qt then treats that palette as user-defined and does not re-polish it on setStyle(), so the theme's palette remaps (notably BrightText, used by the start-page buttons) are lost. The start page draws its button text with palette(bright-text) over a Base (white) background, giving white-on-white.

Fix: after setStyle(), polish the application palette through the style ourselves and re-apply it (Windows only). This re-runs each theme's polish(), restoring the correct colors for all themes (System, Default, Dark). Verified visually on a packaged Windows build across the three themes.

Validation

Validated on a fork by pushing to its master (which exercises the full matrix incl. publish). All builds are green: Linux, macOS, Windows, Flatpak, Formatting, with 27/27 unit tests passing on Linux. The packaged Windows app was extracted and run to confirm the start page renders correctly in all three themes.