Reliability: slow ≠ failed — SW nav timeout, slow-boot telemetry, de-bot stats

Root cause of the intermittent white screen: the shell HTML is no-cache
(cf-cache-status: DYNAMIC), so every page-open does a synchronous round-trip
to the residential origin before any pixel renders — and the SW's network-first
navigation only fell back to the cached shell on REJECTION, never on slowness.
A stalled fetch meant staring at white with a perfectly good shell in cache.
The boot seatbelt couldn't see it either: it lives inside the HTML that hadn't
arrived yet, so slow boots left no telemetry.

- service-worker: race navigation fetch vs 2.5s grace timer. Network wins →
  fresh HTML as before; timer/5xx/failure → cached shell instantly, network
  response still refreshes the cache in the background. Safe due to the 14-day
  immutable-chunk grace window. Caps the white screen at ~2.5s for repeat
  visitors on any network.
- app.html: beacon `boot-slow: Nms (html Nms) on 4g` when mount takes >4s —
  the "white screen, then it loaded" glitches finally leave a trace, with
  HTML-arrival timing to separate slow-origin from slow-JS.
- admin: bot UAs (HeadlessChrome/bot/spider/crawl/…) excluded from the
  headline "Load errors today" count — throttled crawlers trip the 10s boot
  check routinely (the one recorded error was HeadlessChrome on X11, not a
  phone). Bots stay visible in the list, tagged + dimmed.

Tests: telemetry test extended for bot flag + filtered counts. 223 pytest +
11 vitest green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -458,9 +458,9 @@
         <h2>Recent load errors <span class="count">(last {clientErrors.length})</span></h2>
         <ul class="cerrs">
           {#each clientErrors as e (e.created_at + e.reason)}
-            <li>
+            <li class:bot={e.bot}>
               <span class="ce-when">{fdate(e.created_at)}</span>
-              <span class="ce-reason">{e.reason || '—'}</span>
+              <span class="ce-reason">{e.reason || '—'}{#if e.bot}<span class="ce-bot">bot</span>{/if}</span>
               <span class="ce-path">{e.path || '/'}</span>
               <span class="ce-ua">{e.user_agent}</span>
             </li>
@@ -1198,6 +1198,11 @@
               font-size: 0.82rem; padding: 8px 12px; background: var(--surface); border: 1px solid var(--line); border-radius: 8px; }
   .ce-when { color: var(--muted); white-space: nowrap; }
   .ce-reason { font-family: var(--label); color: #9a3b3b; }
+  .cerrs li.bot { opacity: 0.6; }
+  .cerrs li.bot .ce-reason { color: var(--muted); }
+  .ce-bot { display: inline-block; margin-left: 8px; padding: 1px 8px; border-radius: 999px;
+            background: var(--accent-soft); color: var(--accent-deep);
+            font-size: 0.68rem; font-weight: 600; text-transform: uppercase; letter-spacing: 0.05em; }
   .ce-path { color: var(--accent-deep); white-space: nowrap; }
   .ce-ua { grid-column: 1 / -1; color: var(--muted); font-size: 0.72rem;
            overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }