fix: use runtime detection for Wayland clipboard

[benignobjunior]

Problem

When compiled with USE_WAYLAND_CLIPBOARD=ON, the clipboard only works on Wayland sessions. Running the same binary on X11 (e.g., via XWayland or QT_QPA_PLATFORM=xcb) results in clipboard not working.

Solution

Add runtime detection for the platform name:

• wayland → uses KSystemClipboard (KF6 GuiAddons)
• otherwise → uses QApplication::clipboard() (standard Qt)

Benefits

• Distros can ship a single binary that works on both Wayland and X11
• No separate builds needed for different display servers
• Zero impact on builds without USE_WAYLAND_CLIPBOARD

Testing

Tested on CachyOS (Arch-based) with KDE Plasma 6:

• ✅ Wayland native: clipboard works
• ✅ X11 via QT_QPA_PLATFORM=xcb: clipboard works

[borgmanJeremy]

If KGuiAddons will compile on windows and Macos I would prefer to remove it as a compile time option and always build with it. That would simplify it even more.

If its not cross platform then we can leave it like this.

[benignobjunior]

Hey! Good call. I checked the KGuiAddons source and yeah, it builds on all platforms. KSystemClipboard is always compiled and just uses QClipboard when Wayland isn't available, so no platform issues there.

One thing though, I looked at the Flameshot CI and the Windows/macOS workflows don't install any KDE deps (pure Qt builds). Making KF6GuiAddons a hard dep would break those.

What about auto-detection instead?

find_package(KF6GuiAddons QUIET)
if(KF6GuiAddons_FOUND)
    target_compile_definitions(flameshot PRIVATE USE_WAYLAND_CLIPBOARD=1)
    target_link_libraries(flameshot KF6::GuiAddons)
    message(STATUS "KF6GuiAddons found, Wayland clipboard support enabled")
else()
    message(STATUS "KF6GuiAddons not found, using Qt clipboard")
endif()

No cmake flag needed, if it's there it gets used. Linux distros pick it up automatically, Win/Mac CI stays untouched.