Add an All Days button to the schedule editor

Selects all 7 day toggles in one click instead of clicking each one.
Scoped selectors to .day-toggle[data-day] throughout so the new button
(which has no data-day) can't accidentally get read as a day or toggled
by the generic day-button click handler.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H6TgffxTvYhqxykAbVQg2v

templates/dashboard.html

@@ -60,6 +60,7 @@
         {% for d in days %}
         <button type="button" class="day-toggle" data-day="{{ d }}">{{ day_labels[d] }}</button>
         {% endfor %}
+        <button type="button" class="day-toggle day-toggle-all">All Days</button>
       </div>
     </div>