feat: serial:/size: external matching, exclude Hyprland FALLBACK output

- external list now accepts serial: and size: prefixes; desc: matches
  serial too. Fixes docked-mode deadlock where Hyprland's synthetic
  FALLBACK output (created when the built-in is disabled by a stale
  docked config) was treated as a real external monitor, leaving the
  daemon stuck in docked after undocking.
- modes name resolution gains serial: prefix (resolveSerial).
- add -log-level flag for observability.
- add unit tests for external matching and name resolution.
- update README and example config.

internal/backend/hyprland.go

@@ -147,7 +147,7 @@ func (h *hyprlandBackend) WriteConfig(ctx context.Context, monitors []MonitorCon
 			"error", err)
 		var plain []MonitorConfig
 		for _, m := range monitors {
-			if !strings.HasPrefix(m.Name, "desc:") && !strings.HasPrefix(m.Name, "size:") {
+			if !strings.HasPrefix(m.Name, "desc:") && !strings.HasPrefix(m.Name, "serial:") && !strings.HasPrefix(m.Name, "size:") {
 				plain = append(plain, m)
 			}
 		}