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>

app/renderer/editor.js

@@ -125,8 +125,12 @@ class GuideEditor {
     this.steps = steps;
     this.stepMap = new Map(steps.map((step) => [step.stepId, step]));
     if (!this.shellMounted) this.mountShell();
-    if (!this.selectedStepId || !this.stepMap.has(this.selectedStepId)) {
-      this.selectedStepId = stepId && this.stepMap.has(stepId) ? stepId : (steps[0] && steps[0].stepId) || null;
+    // An explicitly requested step (new capture, added step, restored
+    // neighbour) wins; otherwise keep the current selection if it survived.
+    if (stepId && this.stepMap.has(stepId)) {
+      this.selectedStepId = stepId;
+    } else if (!this.selectedStepId || !this.stepMap.has(this.selectedStepId)) {
+      this.selectedStepId = (steps[0] && steps[0].stepId) || null;
     }
     this.selectedAnnotationId = null;
     this.renderAll();