fix: improve mapped installations and map-source guidance (#183)

Improve mapped installation recovery, deselection, retry feedback, and visual-style and map-source guidance.

src/data/installationFeedback.js

@@ -0,0 +1,19 @@
+/** Explain mapped-site availability without claiming an unobserved overload. */
+export function installationFeedback(stats = {}, now = Date.now()) {
+  const reasons = {
+    rate_limited: 'Overpass rate-limited',
+    timeout: 'Overpass timed out',
+    query_failed: 'Overpass could not complete the query',
+  };
+  const reason = reasons[stats.failureReason] || 'Overpass temporarily unavailable';
+  if (stats.loading) return stats.retrying ? 'Retrying mapped sites…' : 'Fetching mapped sites…';
+  if (stats.retryAt > 0) {
+    const seconds = Math.max(0, Math.ceil((stats.retryAt - now) / 1000));
+    return `${reason} — ${seconds ? `retrying in ${seconds}s` : 'retry pending'}`;
+  }
+  if (stats.status === 'unavailable') return reason;
+  if (stats.status === 'zoom-in') return 'Zoom in to search mapped installations';
+  if (stats.stale) return 'Showing cached mapped sites';
+  if (stats.status === 'idle') return 'Mapped sites not loaded';
+  return 'Mapped sites loaded';
+}