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.).

scripts/update_birdnet.sh

@@ -16,7 +16,10 @@ my_dir=$HOME/BirdNET-Pi/scripts
 
 # Defaults
 remote="origin"
-branch="main"
+# AvianVisitors fork: track our overlay branch by default so the
+# Sunday auto-update cron (when enabled) doesn't switch back to
+# upstream `main` and delete the avian/ directory.
+branch="avian-visitors"
 auto_update=""
 
 while getopts ":r:b:a" o; do