Move mode picker below device status bar; fix invisible calendar lines; add CalDAV calendar support

Calendar rule/grid lines were light gray, which dithers away to
near-invisible on the 6-color e-ink palette -- now black.

CalDAV accounts (Nextcloud, Fastmail, iCloud, ...) can now be linked
alongside the existing single ICS subscription, since one account can
expose several calendars. A frame's Calendar tab now lists calendars
per person rather than one opt-in per person: your own row shows every
calendar you have available with a full add/remove toggle, while other
linked users' rows show only calendars they've included, toggleable
off (mute) but not on -- only a calendar's owner can add it to a
shared frame. FrameCalendar replaces the old single-boolean
UserFrame.calendar_included; existing opt-ins are migrated forward.

server/app/templates/frame_photos.html

@@ -2,7 +2,7 @@
 
 {% block title %}{{ frame.name or "Frame" }} · Photos{% endblock %}
 {% block page_title %}{% include "_frame_name_edit.html" %}{% endblock %}
-{% block head_actions %}{% include "_frame_mode_select.html" %}{% endblock %}
+{% block mode_picker %}{% include "_frame_mode_picker.html" %}{% endblock %}
 
 {% block device_status %}{% include "_device_status_bar.html" %}{% endblock %}
 {% block tabs %}{% include "_frame_tabs.html" %}{% endblock %}