Timers: LTC is listener-only; admin switch, device/channel, grouped settings

Simplify LTC to its one working source — the built-in audio listener — and
make the admin UI reflect it.

Module:
- Remove the stage-display websocket source (stage-ws.js, stagePassword)
  and the /v1/timecode/status API probe — both dead on ProPresenter 21.4.
- Remove the remote-reader ingest (POST /ltc, ingestToken, tools/
  ltc-reader.js): the module owns LTC end to end now. The decoder selftest
  moves to modules/propresenter-timers/ltc.selftest.js so the engine stays
  covered.
- composeLtc collapses to the listener: running/stopped/nosignal, or a
  clear note when disabled / no device / capture failed / starting.
- Manifest: ltcEnabled is now a `switch`; ltcDevice and ltcChannel carry
  showWhen:"ltcEnabled"; ProPresenter and LTC fields carry `group` labels.

Shell (admin) — backward-compatible additions, coordinate with the shell lane:
- New `switch` field type (boolean shown as the enable-toggle control).
- `group`: consecutive fields render under a labeled subsection.
- `showWhen: "<key>"`: a field is shown only while that boolean/switch is
  on (and actually collapses — .field[hidden] override, since .field's
  display:flex outranks the [hidden] UA default).
- applyConfigPatch coerces `switch` as boolean. Documented in MODULE-GUIDE.

Verified: engine selftest (29 checks); listener decodes BlackHole loopback
(running 30fps, ageMs); device picker lists inputs; POST /ltc now 404s;
admin shows two labeled groups, the switch, and device/channel appearing
only when the switch is on (computed display none↔flex confirmed).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/admin/admin.css

@@ -113,6 +113,39 @@ body { overflow: auto; }
 .endpoint-row .ep-sep { color: var(--muted); font-weight: 600; }
 .module-form-empty { color: var(--muted); font-size: 12px; margin-top: 10px; }
 
+/* labeled config subsections (manifest field `group`) */
+.field-group {
+  margin: 14px 0 4px;
+  padding: 12px 14px;
+  border: 1px solid var(--border);
+  border-radius: 9px;
+}
+.field-group-title {
+  font-size: 11px;
+  font-weight: 700;
+  text-transform: uppercase;
+  letter-spacing: 0.06em;
+  color: var(--muted);
+  margin-bottom: 10px;
+}
+.field-group .field:last-child { margin-bottom: 0; }
+
+/* showWhen: a hidden dependent field must actually collapse — `.field`'s
+   own `display:flex` (an author rule) otherwise outranks the [hidden] UA
+   default and the field stays visible. */
+.module-form .field[hidden] { display: none; }
+
+/* a boolean rendered as a toggle (manifest type `switch`) */
+.field.switch-field {
+  flex-direction: row;
+  align-items: center;
+  gap: 10px;
+  color: var(--text);
+  cursor: pointer;
+  max-width: none;
+}
+.field.switch-field .switch-label { font-size: 13px; }
+
 .module-actions {
   display: flex;
   align-items: center;