Migrate to Qt6

[selsta]

This PR migrates Monero GUI from Qt 5 to Qt 6, including the necessary QML, C++, CMake, deployment, CI, and packaging changes.

@Malinero started this effort and provided much of the foundational work that made this migration possible.

The migration is kept as one large commit because its individual parts are tightly coupled and intermediate commits would not compile. Splitting it further would therefore create an artificial history rather than a useful sequence of working changes.

Everyone who commited code changes to this PR is listed as an author, and also thank you to @jpk68 and RavFX for helping with testing.

Closes #4348

[selsta]

Force pushed to implemented all the suggested changes. @jpk68 @thomasbuilds

[thomasbuilds]

QR scanner is still Qt5?

[selsta]

Likely yes, I did not compile with SCANNER enabled, will put it on my to-do.

[jpk68]

The code looks good to me, however I have not yet gone through Docker-related files. I'll wait for the QR code thing to be resolved before approving.

[thomasbuilds]

List of things my AI said (not verified - cherry pick if you want)

• Restore the sacrificial SOCKS5 proxy line in NetworkAccessBlockingFactory (defense-in-depth; the main block is intact)
• ProcessingSplash animator: running: parent.visible on the RotationAnimator resolves through the document root (Animators have no parent), so it spins forever even when hidden
• GradientBackground strokeWidth: 0 → -1 (0 doesn't disable the stroke; default stroke color is white)
• Leftover console.log in LanguageSidebar (lines 88, 177)
• PortableSettings::setPortable copies from the file, racing the QML Settings ~500 ms flush; portableFilePath() is CWD-relative and uncached
• CI: two jobs still on checkout@v1, upload-artifact name: dead under archive: false, ccache installed but never cached, NDK pinned by SHA-1
• Cosmetic: white-theme close/minimize SVGs are dark so colorization can't tint them, InlineButton's white-theme shadow was dropped, MoneroCalendar hover overrides the selected-day highlight, QtQuickControls1 import alias is misleading

[selsta]

Addressed most of the points mentioned above. Also implemented SCANNER support, was only able to test it on macOS.

Tomorrow I will try to extract a couple more changes that are not strictly Qt6 related as separate PRs.

Restore the sacrificial SOCKS5 proxy line in NetworkAccessBlockingFactory (defense-in-depth; the main block is intact)

This was intentional to reduce the amount of dependencies we need when compiling Qt.

[jpk68]

Looks good to me; again, I have not tested Docker-related "code" or the QR scanner functionality.

[selsta]

I will have to do one more force push in the next days to make this Qt 6.8.4 ready (Qt does not offer pre-built binaries anymore due to their LTS licensing agreement with KDE), so it will require changes to CI and Docker build scripts.

[selsta]

Updated Docker to 6.8.4. This should soon be merge ready unless we receive new bug reports.