Add week and month views to the calendar

The calendar now opens in a week view: the day timeline as seven columns
lined up with the week strip. A month grid is also available, and the
chosen view is remembered.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/static/index.html

@@ -54,9 +54,14 @@
       </form>
       <div id="cal-nav" class="cal-nav" hidden>
         <button id="cal-prev" class="btn" type="button" aria-label="Previous day">‹</button>
-        <input id="cal-date" type="date" aria-label="Day">
+        <input id="cal-date" type="date" aria-label="Date">
         <button id="cal-next" class="btn" type="button" aria-label="Next day">›</button>
         <button id="cal-today" class="btn" type="button">Today</button>
+        <div id="cal-mode" class="segmented" role="group" aria-label="Calendar view">
+          <button type="button" data-mode="day">Day</button>
+          <button type="button" data-mode="week">Week</button>
+          <button type="button" data-mode="month">Month</button>
+        </div>
       </div>
       <div id="list" class="list"></div>
     </main>