fix: show real capture date in map popup (timestamp field + invalid-date guard)

map.js read capture.captured_at, but the backend stores the field as
`timestamp`, so every marker popup rendered "Invalid Date". Fall back to
timestamp and guard against unparseable values. Regenerated README popup
screenshot to reflect the fix.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

static/js/map.js

@@ -182,7 +182,9 @@ function createMarker(capture) {
 
 function createPopupContent(capture) {
     const freqMHz = (capture.frequency / 1e6).toFixed(2);
-    const date = new Date(capture.captured_at).toLocaleDateString();
+    const ts = capture.timestamp || capture.captured_at;
+    const parsed = ts ? new Date(ts) : null;
+    const date = parsed && !isNaN(parsed) ? parsed.toLocaleDateString() : 'Unknown';
 
     let deviceInfo = 'Unknown Device';
     if (capture.device_name) {