Serve collage at http://birdnet.local/ (replaces stock index)

install_services.sh now symlinks five frontend files (index.html,
styles.css, apt.js, masks.json, dims.json) from avian/frontend/ into
the Caddy site root. Caddy serves index.html before index.php, so /
shows the collage and BirdNET-Pi's stock UI moves to /index.php
(still linked from the menu drawer).

API shims stay under /avian/api/. apt.js fetches them with
./avian/api/birdnet-api.php?action=X now that the frontend lives at
root instead of /avian/frontend/.

avian/index.html redirect file removed (no longer needed - / serves
the collage directly).

README + menu drawer updated to reflect / as the entry point and
/index.php as the stock-UI link.

Verified in Docker:
  /                        200 text/html (collage, 4 tiles)
  /masks.json              200 (276 KB)
  /avian/api/...           200 (all PHP shims working)
  Caddy serves index.html as default index, index.php still reachable

avian/api/menu.php

@@ -25,8 +25,9 @@ if (getenv('AV_REQUIRE_AUTH') === '1' && empty($_SERVER['HTTP_AUTHORIZATION']))
 
 echo json_encode([
     'items' => [
-        // Stock BirdNET-Pi UI (sits at the site root)
-        ['label' => 'birdnet-pi',  'href' => '/',                            'native' => false],
+        // Stock BirdNET-Pi UI (AvianVisitors took over `/`, so the stock
+        // UI is reachable at /index.php directly)
+        ['label' => 'birdnet-pi',  'href' => '/index.php',                   'native' => false],
         // BirdNET-Pi log view (php served at /views.php)
         ['label' => 'logs',        'href' => '/views.php?view=Log+Out',      'native' => false],
         ['label' => 'system',      'href' => '/views.php?view=Services',     'native' => false],