fix(recording): restore correct display selection after screen unlock

[pagarsky]

Summary

Fixes #277.

On dual-monitor Macs, when an external display sleeps during screen lock, SCShareableContent.displays transiently reports only the built-in display for a couple of seconds after unlock. ScreenRecorder.setupCapture runs inside that window (via resumeRecording(after: 0.5)) and its fallback to content.displays.first selects the built-in, even though the tracker's activeDisplayID still points at the external. Once cachedDisplay = built-in, the tracker's .removeDuplicates() swallows any republish of the same id, so handleActiveDisplayChange and refreshDisplay never re-run — capture stays on the wrong display until the cursor crosses screens for a genuine tracker id change (in the affected user's real workflow, that meant 22 minutes of misclassified capture).

Fix

Three linked changes in ScreenRecorder.swift:

1. setupCapture: when a preferred display id exists but isn't in the current snapshot, don't fall back to content.displays.first. Persist the wanted id as requestedDisplayID and leave cachedDisplay = nil (captureScreenshot already handles nil).
2. refreshDisplay: same principle — if the target id isn't in the snapshot, keep cachedDisplay untouched rather than picking a different display.
3. New observer on NSApplication.didChangeScreenParametersNotification in registerForSleepAndLock() calls refreshDisplay() when screens change. This is the recovery path: when the external monitor finishes waking up, refreshDisplay finds the persisted requestedDisplayID in the fresh snapshot and restores capture to it.

Also removes currentDisplayID != nil from handleActiveDisplayChange's guard, since after (1) .capturing no longer implies a resolved display.

Test plan

• Builds (Debug, macOS arm64)
• Maintainer manual repro: dual-monitor setup, work on external, lock (Ctrl+Cmd+Q), wait 60 s so the external sleeps, unlock, keep working. Resulting timeline card should describe external-monitor content; without the fix it labels as whatever was on the built-in.