Session UX: tray-controlled recording, no per-shot window hiding

Users couldn't click into the app mid-session: every automatic capture
hid the window for the shot, so it vanished under the cursor. Under
WSLg minimize() is a no-op and isFocused() sticks true, so neither can
be used for control.

- Sessions now hide the window once at start and show a red tray icon
  with Capture now / Pause-Resume / Open StepForge (auto-pauses) /
  Finish; finishing or quitting restores/cleans up properly
- Opening the app from the tray pauses capture; resuming tucks the
  window away again
- Automatic captures skip while the cursor is over a visible StepForge
  window (cursor-based, not focus-based, due to WSLg sticky focus)
- Per-shot latency reduced: with the window already hidden the 350 ms
  hide-repaint wait is skipped entirely
- OS notification announces the session; self-tests updated and green

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

CHANGELOG.md

@@ -22,6 +22,12 @@ Initial release.
   click detection or global hotkeys are unavailable (e.g. WSLg/Wayland).
   The REC bar shows the live count and trigger, with Shoot / Auto /
   Pause / Finish controls.
+- Recording sessions tuck the window away once and control everything
+  from a red tray icon (capture now / pause / open / finish) instead of
+  hiding the window for every shot — the app stays reachable
+  mid-session, opening it auto-pauses capture, and per-shot latency
+  drops because the hide-repaint wait is gone. Automatic captures also
+  stand down whenever the cursor is over a visible StepForge window.
 - New captures and newly added steps are now selected in the editor.
 - The app hides its own window during capture so screenshots show your
   work, not StepForge; hotkey captures restore the window without