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>

README.md

@@ -48,10 +48,13 @@ The core workflow:
 - **Guide library** with folders, favorites, title search, full-text search,
   duplicate/move/delete, and a quick-actions palette (`Ctrl+/`).
 - **Capture engine** — the editor's **Capture ▾** button offers full screen,
-  active window, and region capture (the app hides itself during the shot)
-  plus delay, pause/resume sessions with global hotkeys, click markers,
-  clipboard paste, and PNG/JPEG/GIF import. The full keyboard shortcut list
-  lives under **More ▾ → Keyboard shortcuts** in the editor.
+  active window, and region capture (the app hides itself during the shot),
+  plus continuous capture sessions that grab a step on every click where the
+  OS allows it, or on a 3/5/10 s auto-interval everywhere else (the REC bar
+  shows the live count with Shoot/Auto/Pause/Finish controls). Delay, global
+  hotkeys, click markers, clipboard paste, and PNG/JPEG/GIF import included.
+  The full keyboard shortcut list lives under **More ▾ → Keyboard
+  shortcuts** in the editor.
 - **Three-pane editor** — step tree with substeps, statuses
   (todo/in-progress/done), hidden/skipped steps, focused view (zoom/pan that
   never mutates the original image), autosave, and command-stack undo/redo.