Screen recording: add optional max duration

[davidguttman]

Closes #5782.

Adds a Max duration submenu to the screenrecord flow (Unlimited / 15min / 30min / 1h / 2h). "Unlimited" is first and preserves today's behavior. When a finite duration is picked, an auto-stop is scheduled against the specific gpu-screen-recorder PID, so a manual stop (Alt+Print or the waybar indicator) followed by a new recording can't be killed by an orphaned timer — the timer re-checks /proc/$pid before signaling.

The duration prompt happens before omarchy-capture-screenrecording is invoked, so the prompt itself isn't captured. The two helpers (wait_for_screenrecorder_pid, schedule_screenrecord_autostop) keep the call site to a single line and document the safety property.

Test plan

• Unlimited → records until manual stop (unchanged behavior).
• 15min → records and auto-stops at 15min.
• Start 30min, manually stop at ~10s (Alt+Print or waybar click), immediately start a new Unlimited → new recording is not killed at t=30min.
• All four audio/webcam combos still selectable.

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[dhh]

Interesting idea, but can't say it's a problem I've ever had. We have the indicator in the menu to remind you that we're recording. But could be a neat script to add on your own setup, then bind a custom hotkey for it. That's the fun part of owning your own operating system!