Timers: LTC timecode via the stage-display websocket

ProPresenter's HTTP API has no timecode route (verified: 21.4 returns
404), but its classic stage-display websocket still answers — so LTC now
comes from a stage layout field labeled "LTC": a zero-dependency RFC6455
client (stage-ws.js) authenticates (new stagePassword admin config),
resolves the LTC-labeled frame's uid from the layout list, and treats any
message carrying that uid as the timecode text. "Receiving" is a 2.5s
staleness heuristic; frozen or placeholder text reads as no-signal. The
HTTP probe stays authoritative when it ever answers (a 404 verdict is
retried once a minute), and the LTC card surfaces the reason when
timecode is unavailable (bad stage password, no LTC field). pp-timers-mock
grows a /stagedisplay websocket with --stage-pwd and --no-ltc-field to
exercise every path.

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

README.md

@@ -91,8 +91,10 @@ are written by the app itself.
   elapsed) and LTC timecode as auto-sized cards. The ＋ picker lists every
   discovered timer (and the LTC card) as its own tile, or add "All timers"
   and pick a selection via the tile's "Timers ▾" menu; overruns are
-  unmistakably red. Read-only — no start/stop controls. Admin config:
-  ProPresenter endpoint (host : port), password.
+  unmistakably red. LTC is read from a stage-display layout field labeled
+  "LTC" over ProPresenter's stage websocket (its HTTP API has no timecode
+  route). Read-only — no start/stop controls. Admin config: ProPresenter
+  endpoint (host : port), API password, stage display password.
 - **Clock** — a big booth clock; also the smallest possible module and the
   reference for module authors.