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>

docs/MODULE-GUIDE.md

@@ -99,21 +99,35 @@ the app); its settings are kept for a later reinstall.
 Both schemas map field names to specs:
 
 ```
-{ "type": "string" | "number" | "boolean" | "select" | "password" | "endpoint",
+{ "type": "string" | "number" | "boolean" | "switch" | "select" | "password" | "endpoint",
   "label": "Shown next to the field",
   "default": <value>,
+  "group": "Section heading",                         // optional; see below
+  "showWhen": "<other boolean/switch key>",           // optional; see below
   "options": [ { "value": "a", "label": "A" }, … ],  // "select" only
   "optionsRoute": "/devices"                          // "select" only, admin config only
 }
 ```
 
+`boolean` renders as a checkbox; `switch` is the same true/false value shown
+as a toggle (use it for a feature on/off that other fields depend on).
+
+`group` gathers consecutive fields under a labeled subsection in the admin
+page, so a module with several distinct concerns (e.g. a ProPresenter
+connection and an LTC audio input) reads as clearly separated blocks. Fields
+with no `group` render loose, before/after the grouped ones in schema order.
+
+`showWhen: "<key>"` hides a field until the boolean/`switch` field named
+`<key>` is on — e.g. the audio device and channel appear only once the LTC
+listener switch is enabled. The hidden field still keeps its stored value.
+
 A `select` whose choices are only knowable at runtime (audio devices, serial
 ports, discovered sources) sets `optionsRoute` instead of — or as a fallback
 refresh of — static `options`: the admin page GETs
 `/api/modules/<id><optionsRoute>` and expects `{ "options": [ { "value",
 "label" }, … ] }` from one of the module's own server routes. The saved value
 stays selectable even when the route is down or the option has vanished. (See
-`propresenter-timers`' "LTC audio device" for a worked example.)
+`propresenter-timers`' "Audio device" for a worked example.)
 
 `password` fields are admin-config only in practice: their values are stored
 in the server's `modules.json` (in the per-machine data directory, see the