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>

core/settings.js

@@ -18,6 +18,7 @@ const DEFAULT_SETTINGS = {
     hotkeyPauseResume: 'CommandOrControl+Shift+2',
     captureOutsideClicks: true,
     confirmSimpleCapture: false,
+    autoIntervalSec: 5, // session fallback when click capture is unavailable
   },
   editor: {
     focusedViewDefaultForNewSteps: false,