Disambiguate shared calendars with a name colliding with the owner's

If a calendar shared with a user has the same name as one of the user's
own calendars (or as another share from a different owner), DAVx5 would
show two calendars with an identical, indistinguishable title. The
shared calendar's CalDAV display name (Calendar.Name, exposed via
displayname/DAVx5's calendar list) now gets the owning user's username
appended in parentheses in that case, e.g. "personal (alice)" — the
underlying calendar path/URL is unaffected, and a user's own calendars
are never renamed (only shared ones can collide with something else).

internal/caldav/backend.go: calendarMeta split into calendarMeta (own
calendars, unchanged name) and calendarMetaNamed (explicit display name);
new sharedDisplayName(requester, owner, realName) checks requester's own
calendars and other shares for a collision. Wired into both
ListCalendars and GetCalendar (the latter used by direct
PROPFIND/REPORT against a shared calendar's own URL).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/caldav/backend.go

@@ -136,7 +136,8 @@ func (b *Backend) ListCalendars(ctx context.Context) ([]caldav.Calendar, error)
 				continue // owner's calendar no longer exists
 			}
 			localName := sharedCalendarName(sh.Owner, sh.CalendarName)
-			cals = append(cals, b.calendarMeta(sh.Owner, sh.CalendarName, localName))
+			displayName := b.sharedDisplayName(p.Username, sh.Owner, sh.CalendarName)
+			cals = append(cals, b.calendarMetaNamed(sh.Owner, sh.CalendarName, localName, displayName))
 		}
 	}
 
@@ -163,7 +164,11 @@ func (b *Backend) GetCalendar(ctx context.Context, calPath string) (*caldav.Cale
 	if _, err := b.store.GetCollection(owner, "cal-"+realName); err != nil {
 		return nil, webdav.NewHTTPError(http.StatusNotFound, err)
 	}
-	cal := b.calendarMeta(owner, realName, localName)
+	displayName := realName
+	if owner != requester {
+		displayName = b.sharedDisplayName(requester, owner, realName)
+	}
+	cal := b.calendarMetaNamed(owner, realName, localName, displayName)
 	return &cal, nil
 }
 
@@ -369,6 +374,14 @@ func (b *Backend) resolveCalendar(requester, localName string, requireWrite bool
 }
 
 func (b *Backend) calendarMeta(owner, realName, localName string) caldav.Calendar {
+	return b.calendarMetaNamed(owner, realName, localName, realName)
+}
+
+// calendarMetaNamed is calendarMeta with an explicit displayName, used for
+// a calendar shared with the requester whose name collides with one of
+// their own calendars (see sharedDisplayName) — the Description still
+// always names the real calendar, only the client-visible Name changes.
+func (b *Backend) calendarMetaNamed(owner, realName, localName, displayName string) caldav.Calendar {
 	desc := fmt.Sprintf("%s's %s calendar", owner, realName)
 	return caldav.Calendar{
 		Path: calHomePath() + localName + "/",
@@ -376,13 +389,46 @@ func (b *Backend) calendarMeta(owner, realName, localName string) caldav.Calenda
 		// "owner~name" synthetic local name used internally to keep a
 		// shared calendar's URL unique) — clients like DAVx5 show this as
 		// the calendar's label, and "alice~work" looked confusing there.
-		Name:                  realName,
+		Name:                  displayName,
 		Description:           desc,
 		SupportedComponentSet: []string{"VEVENT", "VTODO", "VJOURNAL"},
 		MaxResourceSize:       10 * 1024 * 1024, // 10 MiB
 	}
 }
 
+// sharedDisplayName returns realName, suffixed with the owning user's
+// name in parentheses (e.g. "Work (bob)") if requester has another
+// calendar with the exact same display name — either one of their own,
+// or another share from a different owner — so DAVx5 (and other clients)
+// don't show two calendars with an identical, indistinguishable title.
+// Only called for calendars shared with requester; requester's own
+// calendars never need disambiguating against themselves.
+func (b *Backend) sharedDisplayName(requester, owner, realName string) string {
+	own, err := b.dbase.ListCalendars(requester)
+	if err != nil {
+		b.logger.Warn("listing own calendars for disambiguation", "error", err)
+	}
+	for _, c := range own {
+		if c.Name == realName {
+			return fmt.Sprintf("%s (%s)", realName, owner)
+		}
+	}
+
+	shares, err := b.dbase.CalendarsSharedWith(requester)
+	if err != nil {
+		b.logger.Warn("listing shared calendars for disambiguation", "error", err)
+	}
+	for _, sh := range shares {
+		if sh.Owner == owner && sh.CalendarName == realName {
+			continue // the calendar itself, not a collision
+		}
+		if sh.CalendarName == realName {
+			return fmt.Sprintf("%s (%s)", realName, owner)
+		}
+	}
+	return realName
+}
+
 func (b *Backend) decodeObject(objPath string, data []byte) (*caldav.CalendarObject, error) {
 	// Some clients (Outlook/Exchange, some Thunderbird/Lightning setups)
 	// write Windows timezone names into TZID instead of IANA ones, which