Bring back the big timer; the bar pill takes over on scroll

The big asleep/awake card returns below the frozen bar, and the bar
pill becomes its twin: both tick together, but the pill stays
visibility-hidden — slot reserved, so the bar never shifts — while the
card is on screen, appearing only once the card scrolls out of sight
(rAF-throttled scroll check against the bar's bottom edge).

src/index.html

@@ -78,8 +78,9 @@
 
     <main>
       <section class="day-bar">
-        <!-- Compact asleep/awake counter (the old big clock); hidden until a
-             sleep event exists. Hover/long-press shows "since" via title. -->
+        <!-- Compact twin of the big timer below: invisible (but keeping its
+             slot) while the big card is on screen, shown once it scrolls
+             away. Hidden entirely until a sleep event exists. -->
         <div id="bar-clock" class="bar-clock" hidden>
           <span id="bar-clock-icon" aria-hidden="true"></span>
           <span id="bar-clock-time"></span>
@@ -90,6 +91,12 @@
         <button type="button" id="day-today" class="ghost">Today</button>
       </section>
 
+      <section id="big-clock" class="big-clock" hidden>
+        <div class="bc-label" id="bc-label">—</div>
+        <div class="bc-time" id="bc-time">0:00</div>
+        <div class="bc-since" id="bc-since"></div>
+      </section>
+
       <section class="quick-actions">
         <h2>Log event</h2>
         <div class="grid">