Menu: rename 'avianvisitors' link to 'github' + add built-by attribution

avian/api/menu.php: 4th drawer link label is now 'github' (shorter,
matches what the link actually points to).

avian/frontend/index.html: 'built by teddy' attribution sits below
the menu items (sibling of #dd-items inside #menu-dd) so it shows in
both local and forwarded modes. Drops the redundant 'built with
AvianVisitors' line that lived inside the lock screen - the github
menu item covers source-of-truth attribution.

avian/api/menu.php

@@ -31,7 +31,7 @@ echo json_encode([
         // 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],
-        // AvianVisitors docs + source
-        ['label' => 'avianvisitors', 'href' => 'https://github.com/Twarner491/AvianVisitors', 'native' => false],
+        // AvianVisitors source
+        ['label' => 'github', 'href' => 'https://github.com/Twarner491/AvianVisitors', 'native' => false],
     ],
 ]);