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.js

@@ -102,15 +102,35 @@ function fieldFor(key, spec, value, passwordSet, moduleId) {
   const type = spec?.type || 'string';
   const label = spec?.label || key;
   const field = document.createElement('label');
-  field.className = 'field' + (type === 'boolean' ? ' check' : '');
+  field.className = 'field' + (type === 'boolean' ? ' check' : '') + (type === 'switch' ? ' switch-field' : '');
   let read;
+  // For boolean/switch fields, the control other fields may depend on
+  // (showWhen) — returned so the caller can wire visibility.
+  let control = null;
 
-  if (type === 'boolean') {
+  if (type === 'switch') {
+    // Same toggle as the module enable switch, inline with its label.
+    const toggle = document.createElement('span');
+    toggle.className = 'switch';
+    const input = document.createElement('input');
+    input.type = 'checkbox';
+    input.checked = Boolean(value);
+    const track = document.createElement('span');
+    track.className = 'track';
+    toggle.append(input, track);
+    const text = document.createElement('span');
+    text.className = 'switch-label';
+    text.textContent = label;
+    field.append(toggle, text);
+    read = () => input.checked;
+    control = input;
+  } else if (type === 'boolean') {
     const input = document.createElement('input');
     input.type = 'checkbox';
     input.checked = Boolean(value);
     field.append(input, document.createTextNode(label));
     read = () => input.checked;
+    control = input;
   } else if (type === 'endpoint') {
     // One consistent "Host / IP : Port" control for every module's upstream.
     const span = document.createElement('span');
@@ -185,7 +205,7 @@ function fieldFor(key, spec, value, passwordSet, moduleId) {
     field.append(span, input);
     read = () => (type === 'number' ? Number(input.value) : input.value);
   }
-  return { field, read };
+  return { field, read, control };
 }
 
 function buildModuleCard(mod) {
@@ -298,10 +318,42 @@ function buildModuleCard(mod) {
     const form = document.createElement('div');
     form.className = 'module-form';
     const readers = new Map();
+    const controls = new Map(); // key → boolean/switch input (for showWhen)
+    const dependents = []; // { field, when } to show/hide by a control's state
+    // Fields render in schema order; a `group` starts a labeled subsection
+    // and consecutive same-group fields share it.
+    let groupName = null;
+    let container = form;
     for (const [key, spec] of schemaKeys) {
-      const { field, read } = fieldFor(key, spec, mod.config[key], mod.passwordSet[key], mod.id);
+      const g = spec?.group || '';
+      if (g !== groupName) {
+        groupName = g;
+        if (g) {
+          const group = document.createElement('div');
+          group.className = 'field-group';
+          const title = document.createElement('div');
+          title.className = 'field-group-title';
+          title.textContent = g;
+          group.appendChild(title);
+          form.appendChild(group);
+          container = group;
+        } else {
+          container = form;
+        }
+      }
+      const { field, read, control } = fieldFor(key, spec, mod.config[key], mod.passwordSet[key], mod.id);
       readers.set(key, read);
-      form.appendChild(field);
+      if (control) controls.set(key, control);
+      if (spec?.showWhen) dependents.push({ field, when: spec.showWhen });
+      container.appendChild(field);
+    }
+    // A field with showWhen:"K" is visible only while K's toggle is on.
+    for (const dep of dependents) {
+      const ctl = controls.get(dep.when);
+      if (!ctl) continue;
+      const sync = () => { dep.field.hidden = !ctl.checked; };
+      ctl.addEventListener('change', sync);
+      sync();
     }
     const actions = document.createElement('div');
     actions.className = 'module-actions';