input: allow layout switching and brightness/volume control under session lock

[utrumo]

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Summary

Allow common multimedia keys to work on the lock screen by extending
the early-return whitelist:

• Super+Space — switch keyboard layout
• XF86MonBrightnessUp/Down — display brightness
• XF86AudioRaiseVolume/LowerVolume, XF86AudioMute — audio output

All added actions flow through the existing System handler.

Layout switching is functional standalone (the new layout is applied and
typed-into-password fields use it), but the locker UI (dropdown checkmark
and active-layout label) will not reflect the change without the companion
pop-os/cosmic-greeter#439, which subscribes to xkb_config updates.

Test plan

• Lock the screen (Super+Esc or loginctl lock-session) and verify each key:
  • Super+Space switches layout (functional; visual update requires cosmic-greeter#439)
  • Brightness up/down adjusts display brightness
  • Volume up/down/mute adjusts audio
• Same keys still work in unlocked session
• Other shortcuts under lock (e.g. Super+T) — still blocked

[jackpot51]

The PR template is mandatory

[utrumo]

@jackpot51, I fixed the PR description

[Drakulix]

Why not simply exclude the InputSourceSwitch action from the early return when a lock screen is engaged?

[utrumo]

@Drakulix, you were right, I overcomplicated this too much. Your approach is cleaner. I've rewritten the PR. I also added keys for brightness and volume control.

[Drakulix]

Thanks!

[leviport]

This makes volume up/down/mute, laptop screen brightness up/down, and keyboard layout switch hotkeys work at the lock screen. It does not enable them at the initial login screen, but that's outside of the scope of this PR.

I did notice that the keyboard layout button and drop-down don't update when I change my keyboard layout with the keyboard hotkey. For example, if I have both Dvorak and QWERTY enabled and have Dvorak active, pressing Super + Space changes the layout, but clicking the layout button on the greeter reveals a drop-down with Dvorak still selected, even though QWERTY keystrokes are still being typed. I think this is a cosmic-greeter bug, so I will open an issue in that repo.

[utrumo]

@leviport, I fixed it in this mr:
pop-os/cosmic-greeter#439