Apply the chart window to the timing panel

The 7d/14d/30d picker already drove every chart on the page, but the timing
panel was not a chart and got missed: gapsBetween defaulted to a literal 7 and
the heading said "(last 7 days)" as fixed text, so picking 30d widened the
counts and heatmap while the pee, poo and meal gaps under them stayed on a
week. Two panels describing the same events over different windows is the kind
of disagreement nobody thinks to check for.

The default is now chartDays(), read at call time so it tracks the picker
rather than freezing at load, and the heading joins the other panels in
deriving its "(last N days)" from the same place. The two sentences that
quoted seven days — the empty-row fallback and the range tooltip — say the
picked number, and the note about the faded stretch says "the window" rather
than "the week", which is no longer always true.

An explicit days argument still wins, so a future caller wanting a fixed
window is not forced through the picker.

src/index.html

@@ -191,7 +191,7 @@
       </section>
 
       <section class="timing" data-panel="timing">
-        <h2>Timing <span class="muted-note">(last 7 days)</span></h2>
+        <h2>Timing <span class="muted-note" data-chart-days-label>(last 7 days)</span></h2>
         <!-- One range chart per type, drawn by drawTimingChart: the band spans
              the shortest to the typical gap and the marker is how long it has
              been since the last one, so a marker past the band reads as due. -->
@@ -214,7 +214,7 @@
         </div>
         <!-- The axis is stretched (see drawTimingChart), so say what the middle
              of a bar means rather than leave it to be inferred. -->
-        <p class="muted-note">The middle of every bar is that type's typical gap: left of it is sooner than usual, right of it is longer, and the faded stretch runs out to the longest gap of the week.</p>
+        <p class="muted-note">The middle of every bar is that type's typical gap: left of it is sooner than usual, right of it is longer, and the faded stretch runs out to the longest gap in the window.</p>
         <p class="muted-note timing-hint" id="timing-hint"></p>
       </section>