fix: reroute synthetic media keys (Steam Input Volume +/-)

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Currently, holding steam or QAM and pressing left stick up or down will modify system volume while QAM is visible or while in Steam BPM in a gamescope-session, but while a game is focused this chord does not work.

More details documented here:

ublue-os/bazzite#4856

The core issue is that the reroute key logic in wlserver_handle_key only works for real keyboard events, but synthetic keys from Steam Input go through wlserver_key. and that path would forward those events to the focused game surface, not reroute to root server input surface.

PR here ropes wlserver_key into the same forbidden key route, so synthetic vol keys get handled in the same was as physical ones.

After building with this fix, here's it working with steam controller 2:

IMG_3405.MOV

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Video from pr description was tested with patched upstream gamescope but on bazzite HTPC (-deck) edition, but to further test, here’s current shipped gamescope in steamOS reproducing same issue:

IMG_3420.mov

as you can see, with QAM open the inputs are sent to root server input surface but not while game is focused

so this is not isolated to one set of hardware or a Bazzite quirk. This fixes intended behavior in SteamOS as well

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And here’s on SteamDeck and Steam os with patched gamescope:

IMG_3422.mov

Both QAM focused and in-game both pass through the synth keys correctly

Edit: also confirmed that the controls on the SteamDeck itself still send brightness +/- with steam/QAM + LStick up/down, as it should