Add calendar color support (DAVx5 calendar-color)

Calendars can now have a color (hex, e.g. #3b82f6) that DAVx5 and other
CalDAV clients pick up via the Apple/dav4jvm calendar-color property.

- db: add calendars.color column with migration for existing DBs;
  CreateCalendarWithColor, SetCalendarColor, GetCalendarColor;
  ListCalendars now returns []Calendar{Name, Color} instead of []string
- caldav: since go-webdav's caldav.Backend interface has no extension
  point for vendor properties, wrap the handler with a response-rewriting
  middleware that injects <calendar-color xmlns="http://apple.com/ns/ical/">
  into PROPFIND responses for calendars that have a color set
- web: color picker on the "New calendar" form and an inline color swatch/
  picker on each calendar card (calendars only, not address books)
- nidusctl: `calendar create --color` flag and a new `calendar color`
  subcommand; `calendar list` now also prints the color if set

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

internal/db/users.go

@@ -160,10 +160,24 @@ func (d *DB) UserCount() (int, error) {
 
 // -------- calendars --------
 
+// Calendar is a calendar registration, including its display color.
+type Calendar struct {
+	Name  string
+	Color string
+}
+
 // CreateCalendar registers a new calendar owned by owner. Returns
 // ErrResourceExists if it already exists.
 func (d *DB) CreateCalendar(owner, name string) error {
-	_, err := d.conn.Exec(`INSERT INTO calendars (owner, name) VALUES (?, ?)`, owner, name)
+	return d.CreateCalendarWithColor(owner, name, "")
+}
+
+// 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.
+func (d *DB) CreateCalendarWithColor(owner, name, color string) error {
+	_, err := d.conn.Exec(`INSERT INTO calendars (owner, name, color) VALUES (?, ?, ?)`, owner, name, color)
 	if err != nil {
 		if isUniqueConstraintErr(err) {
 			return ErrResourceExists
@@ -173,6 +187,30 @@ func (d *DB) CreateCalendar(owner, name string) error {
 	return nil
 }
 
+// SetCalendarColor updates a calendar's display color. Returns
+// ErrResourceNotFound if it doesn't exist.
+func (d *DB) SetCalendarColor(owner, name, color string) error {
+	res, err := d.conn.Exec(`UPDATE calendars SET color = ? WHERE owner = ? AND name = ?`, color, owner, name)
+	if err != nil {
+		return fmt.Errorf("setting calendar color: %w", err)
+	}
+	return requireRowsAffected(res, ErrResourceNotFound)
+}
+
+// GetCalendarColor returns a calendar's display color. Returns
+// ErrResourceNotFound if it doesn't exist.
+func (d *DB) GetCalendarColor(owner, name string) (string, error) {
+	var color string
+	err := d.conn.QueryRow(`SELECT color FROM calendars WHERE owner = ? AND name = ?`, owner, name).Scan(&color)
+	if err != nil {
+		if errors.Is(err, sql.ErrNoRows) {
+			return "", ErrResourceNotFound
+		}
+		return "", fmt.Errorf("getting calendar color: %w", err)
+	}
+	return color, nil
+}
+
 // DeleteCalendar removes a calendar registration (not the underlying
 // files/objects — callers are responsible for also removing those via
 // store.Store). Returns ErrResourceNotFound if it doesn't exist. Any
@@ -197,10 +235,23 @@ func (d *DB) DeleteCalendar(owner, name string) error {
 	return tx.Commit()
 }
 
-// ListCalendars returns the names of all calendars owner has registered,
-// sorted.
-func (d *DB) ListCalendars(owner string) ([]string, error) {
-	return listNames(d, `SELECT name FROM calendars WHERE owner = ? ORDER BY name`, owner)
+// ListCalendars returns all calendars owner has registered, sorted by name.
+func (d *DB) ListCalendars(owner string) ([]Calendar, error) {
+	rows, err := d.conn.Query(`SELECT name, color FROM calendars WHERE owner = ? ORDER BY name`, owner)
+	if err != nil {
+		return nil, fmt.Errorf("listing calendars: %w", err)
+	}
+	defer rows.Close()
+
+	var cals []Calendar
+	for rows.Next() {
+		var c Calendar
+		if err := rows.Scan(&c.Name, &c.Color); err != nil {
+			return nil, fmt.Errorf("scanning calendar: %w", err)
+		}
+		cals = append(cals, c)
+	}
+	return cals, rows.Err()
 }
 
 // -------- address books --------