Issue #19 Improve DX Cluster Exclusion (partial)

Modify the existing callsign exclusion to:
1. Only exclude DX stations, instead of both DE and DX
2. Treat the entered callsign as prefix, instead of matching any substring
e.g. excluding "VE" now excludes 'VE3ABC' but not 'K3VE'

src/components/DXFilterManager.jsx

@@ -279,7 +279,7 @@ export const DXFilterManager = ({ filters, onFilterChange, isOpen, onClose }) =>
     <div>
       <div style={{ marginBottom: '16px' }}>
         <div style={{ fontSize: '13px', fontWeight: '600', color: 'var(--text-primary)', marginBottom: '8px' }}>
-          Exclude List - Hide these callsigns
+          Exclude List - Hide DX callsigns beginning with:
         </div>
         <div style={{ display: 'flex', gap: '8px' }}>
           <input

src/hooks/useDXCluster.js

@@ -26,17 +26,16 @@ export const useDXCluster = (source = 'auto', filters = {}) => {
       // Watchlist only mode - must match watchlist
       if (filters.watchlistOnly && filters.watchlist?.length > 0) {
         const matchesWatchlist = filters.watchlist.some(w => 
-          spot.call?.toUpperCase().includes(w.toUpperCase()) ||
-          spot.spotter?.toUpperCase().includes(w.toUpperCase())
+          spot.call?.toUpperCase().includes(w.toUpperCase())
         );
         if (!matchesWatchlist) return false;
       }
       
-      // Exclude list - hide matching calls
+      // Exclude list - hide matching calls - match the call as a prefix
       if (filters.excludeList?.length > 0) {
         const isExcluded = filters.excludeList.some(exc =>
-          spot.call?.toUpperCase().includes(exc.toUpperCase()) ||
-          spot.spotter?.toUpperCase().includes(exc.toUpperCase())
+          spot.call?.toUpperCase().startsWith(exc.toUpperCase()) ||
+          spot.spotter?.toUpperCase().startsWith(exc.toUpperCase())
         );
         if (isExcluded) return false;
       }

src/utils/callsign.js

@@ -242,8 +242,7 @@ export const filterDXPaths = (paths, filters) => {
     // Exclude list - hide matching callsigns
     if (filters.excludeList?.length > 0) {
       const isExcluded = filters.excludeList.some(e =>
-        path.dxCall?.toUpperCase().includes(e.toUpperCase()) ||
-        path.spotter?.toUpperCase().includes(e.toUpperCase())
+        path.dxCall?.toUpperCase().startsWith(e.toUpperCase())
       );
       if (isExcluded) return false;
     }