Add native MapKit clustering to the map tab

## Context
The plan calls for replacing the current SwiftUI `Map` annotations with a native `MKMapView` wrapper so visible libraries can cluster when zoomed out.

## Scope
- Add `LibraryAnnotation` to carry a `Library` with each annotation.
- Add individual and cluster annotation views with clear count styling.
- Build `ClusteredMapView` as a `UIViewRepresentable` around `MKMapView`.
- Keep existing filters, navigation, selected-library sheet, and user location behavior.

## Acceptance criteria
- Zoomed-out maps show cluster bubbles with counts.
- Zooming in splits clusters into individual library pins.
- Tapping a pin still opens the existing library card and detail flow.