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/sway.go

@@ -193,9 +193,9 @@ func (s *swayBackend) GetMonitors(ctx context.Context) ([]MonitorInfo, error) {
 // then disabled monitors are turned off. All commands are sent in a single
 // swaymsg call separated by ';' so Sway executes them as one IPC message.
 //
-// Monitor names prefixed with "desc:" are resolved to actual connector names
-// by querying the current monitor state. This makes configs portable across
-// dock ports and reboots.
+// Monitor names prefixed with "desc:", "serial:", or "size:" are resolved
+// to actual connector names by querying the current monitor state. This
+// makes configs portable across dock ports and reboots.
 //
 // Safety: the daemon guarantees at least one enabled monitor exists before
 // calling ApplyLayout. Sway refuses to disable the last active output, so