Make capture sessions continuous: click-capture + interval auto-capture

The session previously only listened for the global hotkey, which is
unreliable under WSLg/Wayland — users got one screenshot and nothing
more. Sessions now layer three triggers:

- click-capture via OS adapters (xinput test-xi2 on X11, PowerShell
  GetAsyncKeyState polling on Windows), debounced, ignoring clicks on
  StepForge itself
- interval auto-capture (3/5/10 s) as the always-works fallback,
  enabled by default when click detection is unavailable
- the existing global hotkey, plus a manual Shoot button

The REC bar now shows live count + active trigger with Shoot / Auto /
Pause / Finish. New captures and added steps are selected in the
editor (explicit reload(stepId) wins over a surviving selection).
Capture self-test hook (STEPFORGE_CAPTURE_SELFTEST) verifies 3x
hotkey-path captures and interval capture end-to-end.

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

CHANGELOG.md

@@ -16,6 +16,13 @@ Initial release.
 - Capture menu in the editor topbar: full screen / window / region /
   3-second delay, paste image as step, import images, and capture
   session start/finish — capture no longer requires the global hotkey.
+- Continuous capture sessions: steps are grabbed on every OS click where
+  the platform supports it (xinput on X11, PowerShell on Windows), with
+  interval auto-capture (3/5/10 s) as the always-works fallback when
+  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.
+- 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
   stealing focus.