feat(gateway): unsupported-engine telemetry beacon + Grafana counter

The index.html boot guard now fires a fire-and-forget beacon (POST /telemetry/unsupported)
when it turns a client away on the unsupported-engine screen, so the owner can see — on the
Users dashboard beside "app opens" — how many real clients hit it and on which engines.

- Client: navigator.sendBeacon (fetch fallback) with a localStorage dedup keyed by app version
  + reason + Chromium, so a user reopening the app is one report, not ten.
- Gateway: a new unauthenticated POST /telemetry/unsupported handler (the client never booted,
  so it carries no session), per-IP public-limited and body-capped, mirroring the export-download
  route. It folds the beacon into the OTel counter unsupported_engine_total {reason =
  no_bigint/no_proxy/boot_error/other, chromium}, with reason allow-listed and the Chromium major
  reduced to a bounded range (normalizeUnsupported) so a spoofed beacon cannot inflate the metric
  cardinality; the full user agent is logged, not labelled.
- Caddy: /telemetry/* added to the @gateway matcher (else it falls to the landing catch-all).
- Grafana: two panels on the Scrabble — Users dashboard (by reason, by Chromium major).
- Docs: ARCHITECTURE.md §11.

Tests: recordUnsupportedEngine (metric split), normalizeUnsupported (cardinality bounding), and
the handler route end to end (204 / 405 / counter). go build+vet+test and gofmt clean; ui
check/build/e2e green; the client beacon + dedup verified against a forced hard-gate (BigInt
removed) — one POST, deduped on reopen, correct reason/chromium/version labels.

docs/ARCHITECTURE.md

@@ -1070,6 +1070,17 @@ browser), so an open in-app session reflects it at once.
   it fills IndexedDB — and is surfaced on the **Scrabble — Users** dashboard. Lossy by
   design (a dropped beacon just retries its batch on the next flush); it is telemetry, never
   a game input.
+- **Unsupported-engine screens (client-reported):** the ES5 boot guard in `index.html` (§13)
+  shows a full-screen "your device's OS or browser can't run the app" screen — instead of a white
+  screen — when the engine lacks an unpolyfillable essential (`BigInt`, the 64-bit FlatBuffers
+  decode; or `Proxy`, Svelte 5 runes) or an uncaught error aborts boot; the effective floor is
+  Chrome 67. It then fires one fire-and-forget beacon (`POST /telemetry/unsupported` —
+  unauthenticated, since the client never booted, but per-IP public-limited and body-capped),
+  deduped in `localStorage` by app version + reason + Chromium so a user reopening the app is one
+  report. The gateway folds it into `unsupported_engine_total` (`reason` =
+  no_bigint/no_proxy/boot_error/other; `chromium` = the major version reduced to a bounded range so
+  a spoofed beacon cannot inflate cardinality) and logs the full user agent (not a label). Surfaced
+  on the **Scrabble — Users** dashboard beside app opens.
 - **Rate-limit observability:** every limiter rejection increments the gateway
   counter `gateway_rate_limited_total` (`class` = user/public/email/admin — aggregate
   only, honouring the no-per-user-label discipline above) and logs one **Debug** line;