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_feed.py

@@ -1,10 +1,11 @@
-"""Fetch, parse, and merge per-user ICS calendar feeds for calendar frame
-mode (see routers/device.py's RENDERERS["calendar"] and calendar_render.py).
+"""Fetch, parse, and merge per-user calendar feeds -- ICS subscriptions
+and (via caldav_client.py) CalDAV collections -- for calendar frame mode
+(see routers/device.py's RENDERERS["calendar"] and calendar_render.py).
 
 Pure functions -- no ORM, no FastAPI Depends. Callers (routers/common.py's
-get_or_refresh_calendar_events) supply plain (owner_display_name, url)
-pairs, not ORM objects, so this module stays testable against fixture .ics
-text with no database or app involved.
+get_or_refresh_calendar_events) supply plain CalendarSource values, not
+ORM objects, so this module stays testable against fixture .ics text with
+no database or app involved.
 
 Recurring events (RRULE/EXDATE/RDATE, DST-aware) are expanded via
 recurring-ical-events rather than hand-rolled -- that's genuinely fiddly
@@ -16,12 +17,15 @@ code under LGPL terms).
 
 from __future__ import annotations
 
+from dataclasses import dataclass
 from datetime import date, datetime
 
 import httpx
 import icalendar
 import recurring_ical_events
 
+from . import caldav_client
+
 HTTP_TIMEOUT_S = 15.0
 FETCH_MAX_BYTES = 10 * 1024 * 1024  # sanity cap -- a real feed is KB, not MB
 
@@ -83,26 +87,46 @@ def fetch_source_events(url: str, window_start: date, window_end: date) -> list[
     return events
 
 
+@dataclass(frozen=True)
+class CalendarSource:
+    """One calendar to merge in: either a plain ICS subscription (kind
+    "ics", url is the feed itself) or one CalDAV collection (kind
+    "caldav", url is the calendar's own URL, username/password its
+    account credentials) -- see caldav_client.py. owner_display_name
+    tags every event pulled from this source so a merged agenda can show
+    whose event is whose."""
+
+    owner_display_name: str
+    kind: str
+    url: str
+    username: str = ""
+    password: str = ""
+
+
 def merge_events(
-    sources: list[tuple[str, str]], window_start: date, window_end: date
+    sources: list[CalendarSource], window_start: date, window_end: date
 ) -> tuple[list[dict], str]:
-    """sources: [(owner_display_name, ics_url), ...]. Fetches each
-    independently -- one broken feed never blanks another's events.
-    Returns (merged_time_sorted_events, fetch_summary); fetch_summary is
-    "" when every source succeeded, else "N of M calendars unavailable"
-    (never *which* source -- naming whose feed is down to everyone who
-    looks at a shared household display is a bigger overshare than the
-    outage itself)."""
+    """Fetches each source independently -- one broken feed never blanks
+    another's events. Returns (merged_time_sorted_events, fetch_summary);
+    fetch_summary is "" when every source succeeded, else "N of M
+    calendars unavailable" (never *which* source -- naming whose feed is
+    down to everyone who looks at a shared household display is a bigger
+    overshare than the outage itself)."""
     merged: list[dict] = []
     failures = 0
-    for owner_display_name, url in sources:
+    for source in sources:
         try:
-            events = fetch_source_events(url, window_start, window_end)
-        except CalendarFetchError:
+            if source.kind == "caldav":
+                events = caldav_client.fetch_calendar_events(
+                    source.url, source.username, source.password, window_start, window_end
+                )
+            else:
+                events = fetch_source_events(source.url, window_start, window_end)
+        except (CalendarFetchError, caldav_client.CalDavError):
             failures += 1
             continue
         for event in events:
-            event["owner_display_name"] = owner_display_name
+            event["owner_display_name"] = source.owner_display_name
             merged.append(event)
 
     merged.sort(key=lambda e: e["start"])