feat(telemetry): local move-preview adoption metrics (Phase 4)

Measure uptake of the client-side local move-preview accelerator (§5) so
adoption can be watched before defaulting it on: app cold starts, dictionary
loads by result (fetched / cache_hit / miss) and move previews by path
(local / network — the backend load shed).

A small best-effort client beacon (POST /metrics/local-eval, session-gated)
batches counter deltas and posts them on a 60s timer and when the app is
backgrounded — never on the gameplay path: the in-app counters are plain
in-memory increments, only the periodic flush touches the network and it is
fire-and-forget. The gateway folds each batch into three OTel counters
(local_eval_cold_start_total, local_eval_dict_load_total,
local_eval_preview_total), clamped against a spoofed inflation.

- gateway: counters + recordLocalEval + session-gated /metrics/local-eval handler
- ui: localeval-metrics accumulator/beacon; hooks in the dict loader, in
  Game.recompute and in bootstrap (skipped under the mock harness)
- caddy: route /metrics/* to the gateway
- docs: ARCHITECTURE §11; Grafana "Scrabble — Users" dashboard panels

ui/src/lib/app.svelte.ts

@@ -9,6 +9,7 @@ import { GatewayError } from './client';
 import { navigate, router } from './router.svelte';
 import { errorKey, localeFrom, setLocale, t, type Locale } from './i18n/index.svelte';
 import { languageNeedsServerSync } from './language';
+import { startLocalEvalMetrics } from './localeval-metrics';
 import { applyReduceMotion, applyTelegramTheme, applyTheme, type ThemePref, type TelegramThemeParams } from './theme';
 import {
   insideTelegram,
@@ -637,6 +638,10 @@ function applyTelegramChrome(launch: TelegramLaunch): void {
 const TELEGRAM_SDK_TIMEOUT_MS = 10000;
 
 export async function bootstrap(): Promise<void> {
+  // Record the cold start and start the adoption beacon (skipped under the mock harness, which
+  // has no real backend). Deliberately before any await, so even an early-return launch path
+  // (e.g. the Telegram launch-error screen) still counts as an app open.
+  if (import.meta.env.MODE !== 'mock') startLocalEvalMetrics();
   const prefs = await loadPrefs();
   app.theme = prefs.theme ?? 'auto';
   app.reduceMotion = prefs.reduceMotion ?? false;