feat: add 18 JSDF bases to military bases layer

Add ASDF (8), MSDF (6), and GSDF (4) bases to military_bases.json.
Colocated bases (Misawa, Yokosuka, Sasebo) have offset coordinates
to avoid overlap with existing US entries. Add branchLabel entries
for GSDF/MSDF/ASDF in MaplibreViewer popup.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/MaplibreViewer.tsx

@@ -1890,6 +1890,7 @@ const MaplibreViewer = ({ data, activeLayers, onEntityClick, flyToLocation, sele
                     if (!base) return null;
                     const branchLabel: Record<string, string> = {
                         air_force: 'AIR FORCE', navy: 'NAVY', marines: 'MARINES', army: 'ARMY',
+                        gsdf: 'GSDF (陸自)', msdf: 'MSDF (海自)', asdf: 'ASDF (空自)',
                     };
                     return (
                         <Popup