Pre-share fixes from final review pass

Blockers
- http://birdnet.local/avian/ now serves the collage (added avian/index.html
  redirect to frontend/; previously the symlink target had no top-level
  index, so users saw a directory listing of api/ assets/ forwarding/
  frontend/ scripts/).
- Gemini model bumped from gemini-2.5-flash-image-preview (deprecated
  2026-01-15) to the stable gemini-2.5-flash-image. Pregen was non-
  functional out of the box.
- avian-mqtt.service: dropped %h (which expands to /root for system-mode
  units regardless of User=). Use WorkingDirectory=~ which DOES respect
  User=, plus a relative ExecStart.
- birdnet-api.php case 'lifelist' now aliases COUNT(*) AS n (was 'total'),
  matching what apt.js reads for atlas card counts. Atlas previously
  always displayed 0.

Bugs
- Retry-After parsing handles HTTP-date format without crashing.
- apt.js no longer fetches firstseen/timeseries (declared in state but
  never consumed). Two wasted round-trips per page load removed.
- update_birdnet.sh defaults branch=avian-visitors so the (opt-in)
  Sunday auto-update cron doesn't reset HEAD to upstream main and
  delete the avian/ overlay.
- recording.php: removed dead $BIRDS_JSON_CANDIDATES.

Polish
- Pregen --sleep default raised 4→6s (more headroom under free-tier RPM).
- en-dash in README BOM (~$35–80) replaced with hyphen for style match.
- Slop sweep across user-facing docs: no em-dashes, no en-dashes, no
  AI-slop word hits (robust/comprehensive/seamless/etc.).

avian/frontend/index.html

@@ -56,7 +56,7 @@
     <button type="button" class="modal-close" data-close="1" aria-label="close">×</button>
     <p class="about-eyebrow">apartment birds</p>
     <h2 id="aboutTitle">The birds outside my window</h2>
-    <p class="about-body">A tiny microphone listens for birds passing the window and identifies them with Cornell's <a href="https://birdnet.cornell.edu/" target="_blank" rel="noopener">BirdNET</a>. Each species shows up as an illustration in the collage — sized by how much it's been heard.</p>
+    <p class="about-body">A tiny microphone listens for birds passing the window and identifies them with Cornell's <a href="https://birdnet.cornell.edu/" target="_blank" rel="noopener">BirdNET</a>. Each species shows up as an illustration in the collage - sized by how much it's been heard.</p>
     <button type="button" class="about-explore" data-close="1">explore the birds →</button>
     <p class="built-by">built with <a href="https://github.com/Twarner491/AvianVisitors" target="_blank" rel="noopener">AvianVisitors</a></p>
   </div>