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/api/spectrogram.php

@@ -1,7 +1,7 @@
 <?php
-// AvianVisitors — serves the spectrogram PNG that BirdNET-Pi generates
+// AvianVisitors - serves the spectrogram PNG that BirdNET-Pi generates
 // alongside each detection mp3. Same lookup logic as recording.php (find
-// the matching file under By_Date/<date>/<Common_Name>/) — just .png
+// the matching file under By_Date/<date>/<Common_Name>/) - just .png
 // instead of .mp3.
 //
 // Endpoints:
@@ -10,7 +10,7 @@
 //                                recording (atlas modal uses this so the
 //                                strip below each play button is the
 //                                spectrogram for that recording, not
-//                                "the most recent" — they can differ).
+//                                "the most recent" - they can differ).
 
 declare(strict_types=1);
 
@@ -44,7 +44,7 @@ if ($file !== '') {
         exit;
     }
     // BirdNET-Pi names the spectrogram as the FULL mp3 filename plus
-    // ".png" — e.g. "American_Crow-82-…-20:25:29.mp3" pairs with
+    // ".png" - e.g. "American_Crow-82-…-20:25:29.mp3" pairs with
     // "American_Crow-82-…-20:25:29.mp3.png" (not "…-20:25:29.png").
     // Accept either form gracefully.
     if (substr($file, -4) === '.png') {