Warm the frame recorder before hiding the window at recording start

The first screenshot of a session was late while every later one was fine.
Cause: on 'Start recording' the window hid first and the capture backend
started warming up after — creating worker, getUserMedia, first frame takes
~1s. A click in that gap found no buffered frame and took the post-click
fresh shot.

armRecording() now warms the recorder while the window is still visible and
only hides once frames are buffering (with a brief post-hide settle so the
first frame shows the user's screen, not the dismissed app window). Verified
end to end with a new self-test scenario that clicks 250ms after start: the
first click is now served a pre-click frame instead of a post-click shot.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

core/settings.js

@@ -33,6 +33,10 @@ const DEFAULT_SETTINGS = {
     // stream pixels lag slightly; a small lead keeps the saved screenshot
     // clear of the click's onset. Raise it if screenshots still feel late.
     clickLeadMs: 120,
+    // After the window hides at recording start, wait this long before the
+    // user is likely to click so the buffer holds frames of the now-visible
+    // screen rather than the just-dismissed app window.
+    postHideSettleMs: 150,
   },
   editor: {
     focusedViewDefaultForNewSteps: false,