fix(review): make WebGL toggle per-device and fix sticky-marker semantics (PR #140)

- saveAppSettings no longer sends webglRendererEnabled on the settings PUT:
  the key is absent from the .strict() SettingsUpdateSchema, so every save
  400'd with INVALID_INPUT, silently killing all server-side settings
  persistence. Stripped in the per-device destructure alongside
  localEchoEnabled/skin/etc.
- shouldSkipWebGL now treats a stored true like the untouched default w.r.t.
  the sticky marker: the checkbox defaults checked on desktop, so any
  unrelated save stored true and every page load then cleared the
  'codeman-webgl-disabled' marker, permanently defeating the GPU-stall
  auto-fallback. Only ?webgl=force clears the marker at init.
- The marker is instead retired on a real OFF->ON toggle flip detected at
  save time (mirrors the _prevGestureEnabled pattern in settings-ui.js).
- webglRendererEnabled added to the displayKeys per-device set in
  loadAppSettingsFromServer (renderer choice is device/GPU-specific; syncing
  would leak mobile's hidden-checkbox false onto desktop).
- Tests: stored true + sticky marker -> still skips WebGL; OFF->ON save
  clears the marker and keeps the key off the wire; default-checked save
  leaves the marker alone; ?webgl=force / ?nowebgl behavior unchanged.

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

src/web/public/terminal-ui.js

@@ -272,7 +272,9 @@ Object.assign(CodemanApp.prototype, {
       stickyDisabled: _stickyDisabled,
       userPrefEnabled: _webglPref,
     });
-    // Explicit opt-in (toggle ON) or ?webgl=force retires a stale auto-fallback marker.
+    // Only ?webgl=force retires the auto-fallback marker at init — a stored
+    // toggle ON is incidental (checkbox defaults checked) and must not defeat
+    // the sticky safety net. An OFF→ON flip clears it in saveAppSettings().
     if (_clearWebglSticky) {
       try { localStorage.removeItem('codeman-webgl-disabled'); } catch {}
     }