fix(web): statusline tokens, unreachable dot, reachability test spans (#145)

Second review round (spec + standards):
- Polling-off and unreachable render as statusline tokens; the unreachable
  span drops mark-strong so the patina dot never sits next to red text.
- Reachability tests assert on the rendered span, which 'reachable' and
  'unreachable' substrings never could.
- A pass-log query failure now reads as reachable (no evidence rule) instead
  of condemning the browser, and admin.go loses its dead time import.
- startLatestPoller's doc no longer claims the admin page reads the poller;
  AGENTS.md carries the RefuseBackoff rename.

backend/internal/web/templates/lanes.html

@@ -14,9 +14,8 @@
   <div class="sechead">
     <h2 class="sec">Poll Lanes</h2>
     <p class="statusline">
-      {{if .PollerOff}}Polling is switched off in this deployment: no Lane
-      runs, and Latest Chapter comes from the userscripts alone.
-      {{else}}Browser: {{if not .BrowserConfigured}}<span class="mark-faint">not configured</span>{{else if .BrowserReachable}}<span class="mark mark-strong">reachable</span>{{else}}<span class="mark mark-strong bad">unreachable</span>{{end}}{{end}}
+      {{if .PollerOff}}Polling: <span class="mark-faint">off</span>
+      {{else}}Browser: {{if not .BrowserConfigured}}<span class="mark-faint">not configured</span>{{else if .BrowserReachable}}<span class="mark mark-strong">reachable</span>{{else}}<span class="mark bad">unreachable</span>{{end}}{{end}}
     </p>
   </div>
   {{if .Rows}}