Add TODAY window mode with configurable start hour

New 'TODAY' button in the window picker shows birds since a
configurable start hour (default 6 AM). The window grows through
the day and resets each morning.

Start hour is adjustable in the settings panel ('Today starts at'
slider) and persists in localStorage (bird:todayStart).

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

avian/frontend/index.html

@@ -27,6 +27,7 @@
   <div class="window-pick" id="winPick" role="tablist">
     <i class="seg-pill" aria-hidden="true"></i>
     <button data-h="1"   type="button">1H</button>
+    <button data-h="today" type="button">TODAY</button>
     <button data-h="12"  type="button">12H</button>
     <button data-h="24"  type="button" aria-current="true">24H</button>
     <button data-h="168" type="button">7D</button>