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/caldav_client.py

@@ -0,0 +1,108 @@
+"""CalDAV account support: discovering which calendars an account exposes,
+and fetching one calendar's events -- the second way (alongside
+calendar_feed.py's single-file ICS subscription) a user can link a
+calendar for calendar frame mode (Nextcloud, Fastmail, iCloud, Radicale,
+Baikal, ...).
+
+Thin wrapper around the `caldav` PyPI package (RFC 4791 client). NOTE ON
+LICENSING: `caldav` itself is dual-licensed GPL-3.0-or-later / Apache-2.0,
+but it hard-depends on `icalendar-searcher`, which is AGPL-3.0-or-later --
+the strongest copyleft license in this project's dependency tree, and the
+one whose network-use clause is specifically written for server
+applications like this one. This was an explicit, informed call by the
+project owner to accept that exposure rather than hand-roll a CalDAV
+client -- see the server README's Notes section. Anyone redistributing
+this project (as opposed to just self-hosting it) should reread that
+tradeoff for their own situation.
+
+Pure functions -- no ORM, no FastAPI Depends -- same testability
+philosophy as calendar_feed.py. Event parsing/expansion reuses
+icalendar + recurring_ical_events directly (rather than trusting each
+CalDAV server's own possibly-inconsistent RRULE expansion) so a CalDAV
+calendar and an ICS subscription behave identically once fetched.
+"""
+
+from __future__ import annotations
+
+from datetime import date, datetime, time as dtime
+
+import caldav
+import icalendar
+import recurring_ical_events
+
+HTTP_TIMEOUT_S = 15
+
+
+class CalDavError(Exception):
+    """Discovery or fetch failed -- network, auth, or an unexpected
+    server response. Raised loudly; callers (Settings' discover
+    endpoint, calendar_feed.merge_events) decide what to do. Wraps
+    whatever the caldav package/its transport raised, since that
+    exception hierarchy isn't something call sites should need to know
+    about directly."""
+
+
+def discover_calendars(base_url: str, username: str, password: str) -> list[dict]:
+    """[{"href": absolute_calendar_url, "display_name": str}, ...] for
+    every calendar in this account. base_url is the server's CalDAV
+    entry point (e.g. "https://cloud.example.com/remote.php/dav/" for
+    Nextcloud) -- the caller supplies it directly, same idiom as the
+    plain ICS subscription URL."""
+    try:
+        client = caldav.DAVClient(url=base_url, username=username, password=password, timeout=HTTP_TIMEOUT_S)
+        calendars = client.principal().calendars()
+    except Exception as e:
+        raise CalDavError(str(e)) from e
+
+    result = []
+    for cal in calendars:
+        try:
+            display_name = cal.get_display_name() or cal.name
+        except Exception:
+            display_name = None
+        result.append({"href": str(cal.url), "display_name": display_name or str(cal.url)})
+    return result
+
+
+def fetch_calendar_events(calendar_url: str, username: str, password: str,
+                           window_start: date, window_end: date) -> list[dict]:
+    """One CalDAV calendar's events in [window_start, window_end] -- same
+    event dict shape as calendar_feed.fetch_source_events (no
+    "owner_display_name"; the caller adds that). Fetches raw (unexpanded)
+    calendar objects and runs them through the same icalendar +
+    recurring_ical_events pipeline calendar_feed.py uses for ICS feeds,
+    rather than relying on server-side expand (RFC 4791 leaves plenty of
+    corner cases server implementations disagree on)."""
+    try:
+        client = caldav.DAVClient(url=calendar_url, username=username, password=password, timeout=HTTP_TIMEOUT_S)
+        calendar = caldav.Calendar(client=client, url=calendar_url)
+        objects = calendar.date_search(
+            start=datetime.combine(window_start, dtime.min),
+            end=datetime.combine(window_end, dtime.min),
+            expand=False,
+        )
+    except Exception as e:
+        raise CalDavError(str(e)) from e
+
+    events: list[dict] = []
+    for obj in objects:
+        try:
+            ical = icalendar.Calendar.from_ical(obj.data)
+            occurrences = recurring_ical_events.of(ical).between(window_start, window_end)
+        except Exception:  # one malformed resource shouldn't blank the whole calendar
+            continue
+        for occ in occurrences:
+            dtstart = occ.get("DTSTART")
+            dtend = occ.get("DTEND")
+            if dtstart is None:
+                continue
+            start_dt = dtstart.dt
+            end_dt = dtend.dt if dtend is not None else start_dt
+            all_day = not isinstance(start_dt, datetime)
+            events.append({
+                "summary": str(occ.get("SUMMARY") or "(untitled)"),
+                "start": start_dt.isoformat(),
+                "end": end_dt.isoformat(),
+                "all_day": all_day,
+            })
+    return events