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/calendar_render.py

@@ -35,7 +35,9 @@ MARGIN = 20
 BG = (255, 255, 255)
 FG = (0, 0, 0)
 MUTED = (110, 110, 110)
-RULE = (200, 200, 200)
+# Was a light gray, but that dithers away to near-invisible once quantized
+# to the 6-color e-ink palette -- black reads as an actual line on-panel.
+RULE = (0, 0, 0)
 
 # Cycled per distinct owner_display_name so a merged multi-person calendar
 # can visually tell whose event is whose -- the panel's own non-black/