Add calendar frame mode + server-side manage overlay (server)

calendar_feed.py/calendar_render.py: fetch/merge per-user ICS feeds,
render agenda/week/month views. manage_overlay.py: composites the
manage-button overlay server-side (QR, battery, location/date,
share-QR, face labels), reused by every render mode. device.py/common.py
wire both together: mode dispatch for /frame/image+advance+back, and
the &manage=1 flag. Plus UI (frame_config.html Calendar card, settings
calendar URL field) and the icalendar/recurring-ical-events deps.

server/app/templates/settings.html

@@ -32,6 +32,19 @@
         this Immich library. The key needs read access to albums/assets/faces
         plus <code>sharedLink.create</code> for the on-frame share QR.</p>
 
+      <h2 class="card-title" style="margin-top: 24px;">Calendar</h2>
+      <label>Calendar URL (iCal/CalDAV .ics feed)
+        <input type="text" name="calendar_ics_url" placeholder="https://calendar.example.com/you.ics"
+               value="{{ user.calendar_ics_url }}">
+      </label>
+      <p class="sub" style="margin-top: 8px;">Your personal calendar
+        subscription link (no login needed -- e.g. Google Calendar's
+        Settings &rarr; "Secret address in iCal format", or Apple/Outlook/
+        Nextcloud's equivalent). Setting it here doesn't show it anywhere
+        by itself -- include it on any frame you're linked to from that
+        frame's Configuration &rarr; Calendar card, so a frame only shows
+        calendars people have actually chosen to share with it.</p>
+
       <h2 class="card-title" style="margin-top: 24px;">Change password</h2>
       <label>Current password
         <input type="password" name="current_password" autocomplete="current-password">