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/AGENTS.md

@@ -134,7 +134,7 @@ unchanged read is exactly the Sighting worth deferring a Poll on.
   rest.
 
 **Refusals and browser loss are Lane-local.** Two `errChallengeHeld` in a pass
-stop that Site for `refuseBackoff` while other Lanes continue. An
+stop that Site for `RefuseBackoff` while other Lanes continue. An
 `errBrowserInterrupted` (remote Chrome restarted) sets a shared Poller flag so
 the *other* browser Lanes skip their passes for the same window — otherwise a
 restarting Chrome stamps one Series per Lane per pass, burning rests on