feat(app): cluster map spots by zoom with leaflet.markercluster

Replace per-moveend circleMarker redraw (300-spot cap) with a single
L.markerClusterGroup containing all 1,553 breakwater spots.
Zoom-out shows regional count bubbles; zoom-in shows individual
circleMarkers. Remove layerRef and moveend listener — clustering is
handled entirely by the library. CSS imports added for cluster icons.

Constraint: circleMarker appearance and tooltip behaviour unchanged.
Confidence: high
Scope-risk: local
Reversibility: trivial
Directive: verify cluster bubble appearance on device build
Tested: tsc + vite build pass (0 type errors); 640 modules transformed
Not-tested: runtime rendering on physical Android device
Related: MEMORY project_sea_and_catch

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