SUMMARY
There is no way to configure handlers for URIs such as "matrix:r/..." or "tg:/" (telegram)

STEPS TO REPRODUCE
1. In Systemsettings open "Apps & Windows > Default Applications > File associations"
2. Look for x-scheme-handler

OBSERVED RESULT
x-scheme-handler is not present

EXPECTED RESULT
x-scheme-handler should be present and all known schemes should be listed when expanding the chevron. "Known schemes" should be at least all those for which at least one app has claimed support (see e.g. `grep x-scheme-handler /usr/share/applications/mimeinfo.cache`)

SOFTWARE/OS VERSIONS
Operating System: Fedora Linux 42
KDE Plasma Version: 6.4.2
KDE Frameworks Version: 6.15.0
Qt Version: 6.9.1
Kernel Version: 6.15.3-200.fc42.x86_64 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 7840U w/ Radeon  780M Graphics
Memory: 32 GiB of RAM (27.2 GiB usable)
Graphics Processor: AMD Radeon 780M Graphics