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/db.go

@@ -64,6 +64,7 @@ CREATE TABLE IF NOT EXISTS users (
 CREATE TABLE IF NOT EXISTS calendars (
 	owner TEXT NOT NULL REFERENCES users (username) ON DELETE CASCADE,
 	name TEXT NOT NULL,
+	color TEXT NOT NULL DEFAULT '',
 	created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
 	PRIMARY KEY (owner, name)
 );
@@ -106,5 +107,43 @@ CREATE TABLE IF NOT EXISTS web_sessions (
 CREATE INDEX IF NOT EXISTS idx_web_sessions_expires_at ON web_sessions (expires_at);
 `
 	_, err := d.conn.Exec(schema)
-	return err
+	if err != nil {
+		return err
+	}
+	return d.migrateAddColumns()
+}
+
+// migrateAddColumns adds columns to already-existing tables that predate
+// their introduction. CREATE TABLE IF NOT EXISTS above only creates a
+// table's initial shape, so columns added later (like calendars.color)
+// need an explicit ALTER TABLE for databases created before this change.
+func (d *DB) migrateAddColumns() error {
+	hasColumn := func(table, column string) (bool, error) {
+		rows, err := d.conn.Query(`SELECT name FROM pragma_table_info(?)`, table)
+		if err != nil {
+			return false, err
+		}
+		defer rows.Close()
+		for rows.Next() {
+			var name string
+			if err := rows.Scan(&name); err != nil {
+				return false, err
+			}
+			if name == column {
+				return true, nil
+			}
+		}
+		return false, rows.Err()
+	}
+
+	ok, err := hasColumn("calendars", "color")
+	if err != nil {
+		return fmt.Errorf("checking calendars.color column: %w", err)
+	}
+	if !ok {
+		if _, err := d.conn.Exec(`ALTER TABLE calendars ADD COLUMN color TEXT NOT NULL DEFAULT ''`); err != nil {
+			return fmt.Errorf("adding calendars.color column: %w", err)
+		}
+	}
+	return nil
 }