feat(overlay): let the user pick which monitor shows the overlay

[Roddygithub]

Closes #21

What

Adds a monitor setting so the overlay can be pinned to a specific
display instead of the compositor's default.

Value	Behaviour
primary (default)	first monitor in GDK's list
cursor / active	monitor under the pointer, resolved live
index:N	Nth monitor, wraps around

How

• MonitorConfig enum + resolve_monitor() in layer_shell.rs, applied
  with gtk4_layer_shell::set_monitor() at startup and re-resolved on every
  settings message, so changing the setting moves the overlay live.
• Cursor detection uses seat.pointer().surface_at_position() +
  display.monitor_at_surface() — no manual geometry math.
• OverlaySettings.monitor is #[serde(default)] so older plugin builds
  still deserialize (default Primary); is_valid() only accepts known
  monitor values so a malformed string never reaches set_monitor().
• Plugin gets a monitor SELECT (primary / cursor / index 0-3) sent in
  sendSettings().

Gates

cargo fmt --check && cargo clippy --all-targets --all-features -- -D warnings && cargo test   → 40 passed
cd plugin && npm run lint && npm test                                                          → 25 passed

memory-bank/progress.md updated per AGENTS.md.