Make birthdays calendar color configurable and expose it via CalDAV

- Add a per-user birthday_calendars table (color, cascade-deletes with
  the user) and GetBirthdayCalendarColor/SetBirthdayCalendarColor in
  internal/db, plus a reservedCalendarNames guard ("birthdays") in
  CreateCalendarWithColor so no real calendar can collide with the
  synthetic one, whether created via the web UI, nidusctl, or CalDAV
  MKCALENDAR.
- Add a "Birthdays" virtual resource card to the dashboard (color
  picker only, no delete/share controls) backed by a new
  ResourceCard.Virtual flag and POST /web/resources/birthdays/color
  handler.
- Extract the birthday-parsing/generation logic shared by the web
  calendar view and CalDAV into internal/birthdays (ParseBirthday,
  Collect, OccurrenceDate, Summary) instead of duplicating it.
- Expose the Birthdays calendar over real CalDAV in
  internal/caldav/backend.go + birthdays.go: it's always listed for
  every user, generates one VEVENT per (contact, year) for a rolling
  window (current year -2..+8) with "🎂 Name (Age)" titles, is
  read-only (Put/Delete/DeleteCalendar all return 403), and its
  Apple/DAVx5 calendar-color is injected from the same per-user
  setting used by the dashboard/web view.

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

internal/db/users.go

@@ -24,6 +24,19 @@ var ErrResourceExists = errors.New("resource already exists")
 // that doesn't exist.
 var ErrResourceNotFound = errors.New("resource not found")
 
+// ErrReservedName is returned when trying to create a calendar whose name
+// is reserved for a computed/virtual calendar (see reservedCalendarNames).
+var ErrReservedName = errors.New("calendar name is reserved")
+
+// reservedCalendarNames are calendar names that can't be used for a real,
+// user-created calendar because they're reserved for a synthetic,
+// computed calendar shown alongside real ones (e.g. "birthdays", see
+// internal/web/calendar.go and internal/caldav/backend.go). Matched
+// case-insensitively.
+var reservedCalendarNames = map[string]bool{
+	"birthdays": true,
+}
+
 // User is an account stored in the database.
 type User struct {
 	Username     string
@@ -175,8 +188,11 @@ func (d *DB) CreateCalendar(owner, name string) error {
 // CreateCalendarWithColor registers a new calendar owned by owner with the
 // given color (an empty string means no color is set, in which case the
 // client picks its own default). Returns ErrResourceExists if it already
-// exists.
+// exists, or ErrReservedName if name is reserved for a computed calendar.
 func (d *DB) CreateCalendarWithColor(owner, name, color string) error {
+	if reservedCalendarNames[strings.ToLower(name)] {
+		return ErrReservedName
+	}
 	_, err := d.conn.Exec(`INSERT INTO calendars (owner, name, color) VALUES (?, ?, ?)`, owner, name, color)
 	if err != nil {
 		if isUniqueConstraintErr(err) {
@@ -254,6 +270,34 @@ func (d *DB) ListCalendars(owner string) ([]Calendar, error) {
 	return cals, rows.Err()
 }
 
+// GetBirthdayCalendarColor returns owner's display color for the
+// synthetic "Birthdays" calendar, or "" if they haven't set one (callers
+// should fall back to a built-in default in that case).
+func (d *DB) GetBirthdayCalendarColor(owner string) (string, error) {
+	var color string
+	err := d.conn.QueryRow(`SELECT color FROM birthday_calendars WHERE owner = ?`, owner).Scan(&color)
+	if errors.Is(err, sql.ErrNoRows) {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("getting birthday calendar color: %w", err)
+	}
+	return color, nil
+}
+
+// SetBirthdayCalendarColor sets (or updates) owner's display color for
+// the synthetic "Birthdays" calendar.
+func (d *DB) SetBirthdayCalendarColor(owner, color string) error {
+	_, err := d.conn.Exec(`
+		INSERT INTO birthday_calendars (owner, color) VALUES (?, ?)
+		ON CONFLICT (owner) DO UPDATE SET color = excluded.color`,
+		owner, color)
+	if err != nil {
+		return fmt.Errorf("setting birthday calendar color: %w", err)
+	}
+	return nil
+}
+
 // -------- address books --------
 
 // CreateAddressBook registers a new address book owned by owner. Returns