input: allow InputSourceSwitch under session lock via direct D-Bus call

[utrumo]

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Summary

Allow switching keyboard layouts on the lock screen via Super+Space.

Under session_lock, the System(InputSourceSwitch) action is
handled by calling the InputSourceSwitch D-Bus method on
com.system76.CosmicSettingsDaemon directly.

Test plan

• Lock the screen (Super+Esc or loginctl lock-session) → press
  Super+Space — layout switches, label and dropdown update
  (requires matching cosmic-greeter patch: locker: watch xkb_config for live layout updates and show active layout label cosmic-greeter#439)
• Super+Space in unlocked session — works as before
• Other shortcuts under lock (e.g. Super+T) — still blocked

[jackpot51]

The PR template is mandatory

[utrumo]

@jackpot51, I fixed the PR description