Add location search, home/fit map buttons, and slim OSM attribution

- TopNavBar: Nominatim geocoding search field plus Home (center on first
  waypoint) and Fit (zoom to mission bounds) buttons (HTML prototype:
  #searchWrap/#homeBtn/#fitBtn). Both show a SnackBar instead of doing
  nothing when there are no waypoints yet.
- Renders as its own bar just below TopModeBar rather than fused into the
  same pill: TopModeBar lives in AppShell, a layer above PlanScreen, which
  owns the MapController these actions actually need. Fusing them would
  need lifting the MapController to shared state - reasonable follow-up
  if pixel-fidelity with the prototype's single bar matters later, but
  not necessary for the functionality itself.
- Replaced SimpleAttributionWidget with a compact custom attribution: OSM's
  tile usage policy requires visible attribution to stay, so instead of
  removing it (as first asked) I shrank it and dropped the "flutter_map |"
  prefix per the user's follow-up choice, so it reads cleanly against the
  now-opaque bottom bar instead of looking like a second banner.

Added 5 tests (nav bar renders, home/fit SnackBars with no waypoints,
empty search is a no-op). All 26 tests and flutter analyze pass. Verified
on the Pixel_10a emulator: searched "Berlin" and confirmed the map flew
there, dropped a waypoint, panned far away, and confirmed Home re-centers
and snaps onto it exactly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/lib/ui/widgets/mission_map.dart

@@ -72,13 +72,38 @@ class MissionMap extends StatelessWidget {
           ],
         ),
         CircleLayer(circles: waypointMarkers),
-        SimpleAttributionWidget(
-          alignment: Alignment.bottomLeft,
-          source: const Text('OpenStreetMap contributors'),
-          onTap: () =>
-              launchUrl(Uri.parse('https://www.openstreetmap.org/copyright')),
-        ),
+        const _CompactAttribution(),
       ],
     );
   }
 }
+
+/// Schlanke Pflicht-Attribution fuer die OSM-Kacheln (Tile Usage Policy) -
+/// bewusst ohne den "flutter_map |"-Praefix von SimpleAttributionWidget und
+/// dezenter gestaltet, damit sie nicht wie ein zweiter Balken ueber der
+/// Bottom-Bar wirkt. Bleibt sichtbar und antippbar, da die Policy sichtbare
+/// Namensnennung verlangt.
+class _CompactAttribution extends StatelessWidget {
+  const _CompactAttribution();
+
+  @override
+  Widget build(BuildContext context) {
+    return SafeArea(
+      child: Align(
+        alignment: Alignment.bottomLeft,
+        child: GestureDetector(
+          onTap: () =>
+              launchUrl(Uri.parse('https://www.openstreetmap.org/copyright')),
+          child: Container(
+            padding: const EdgeInsets.symmetric(horizontal: 5, vertical: 1),
+            color: Colors.white.withValues(alpha: 0.5),
+            child: const Text(
+              '© OpenStreetMap contributors',
+              style: TextStyle(fontSize: 9, color: Colors.black87),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}