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),
+            ),
+          ),
+        ),
+      ),
+    );
+  }
+}

app/lib/ui/widgets/top_nav_bar.dart

@@ -0,0 +1,110 @@
+import 'package:flutter/material.dart';
+
+/// Suchleiste + Navigations-Buttons (HTML-Demonstrator: #searchWrap/
+/// #homeBtn/#fitBtn). Rendert unterhalb der Plan/Fly-Kopfleiste statt in
+/// dieselbe Pille eingeschmolzen - TopModeBar liegt im AppShell, eine
+/// Ebene ueber PlanScreen, das den MapController besitzt, den Suche/
+/// Home/Fit tatsaechlich benoetigen. Ein gemeinsamer Riverpod-State fuer
+/// den MapController koennte das spaeter fusionieren.
+class TopNavBar extends StatefulWidget {
+  const TopNavBar({
+    super.key,
+    required this.onSearch,
+    required this.onHome,
+    required this.onFit,
+  });
+
+  final ValueChanged<String> onSearch;
+  final VoidCallback onHome;
+  final VoidCallback onFit;
+
+  @override
+  State<TopNavBar> createState() => _TopNavBarState();
+}
+
+class _TopNavBarState extends State<TopNavBar> {
+  final _controller = TextEditingController();
+
+  @override
+  void dispose() {
+    _controller.dispose();
+    super.dispose();
+  }
+
+  void _submit() {
+    widget.onSearch(_controller.text);
+    FocusScope.of(context).unfocus();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return Row(
+      mainAxisSize: MainAxisSize.min,
+      children: [
+        Container(
+          width: 260,
+          height: 38,
+          padding: const EdgeInsets.only(left: 12, right: 4),
+          decoration: BoxDecoration(
+            color: Colors.black.withValues(alpha: 0.68),
+            borderRadius: BorderRadius.circular(19),
+            border: Border.all(color: Colors.white.withValues(alpha: 0.4)),
+          ),
+          child: Row(
+            children: [
+              Expanded(
+                child: TextField(
+                  controller: _controller,
+                  onSubmitted: (_) => _submit(),
+                  textInputAction: TextInputAction.search,
+                  style: const TextStyle(color: Colors.white, fontSize: 14),
+                  decoration: InputDecoration(
+                    isDense: true,
+                    border: InputBorder.none,
+                    hintText: 'Search location',
+                    hintStyle:
+                        TextStyle(color: Colors.white.withValues(alpha: 0.55)),
+                  ),
+                ),
+              ),
+              IconButton(
+                onPressed: _submit,
+                icon: const Icon(Icons.search, color: Colors.white, size: 18),
+                padding: EdgeInsets.zero,
+                constraints: const BoxConstraints(minWidth: 30, minHeight: 30),
+              ),
+            ],
+          ),
+        ),
+        const SizedBox(width: 8),
+        _NavIconButton(icon: Icons.home, onTap: widget.onHome),
+        const SizedBox(width: 8),
+        _NavIconButton(icon: Icons.crop_free, onTap: widget.onFit),
+      ],
+    );
+  }
+}
+
+class _NavIconButton extends StatelessWidget {
+  const _NavIconButton({required this.icon, required this.onTap});
+
+  final IconData icon;
+  final VoidCallback onTap;
+
+  @override
+  Widget build(BuildContext context) {
+    return GestureDetector(
+      onTap: onTap,
+      child: Container(
+        width: 38,
+        height: 38,
+        decoration: BoxDecoration(
+          shape: BoxShape.circle,
+          color: Colors.black.withValues(alpha: 0.65),
+          border: Border.all(color: Colors.white.withValues(alpha: 0.4)),
+        ),
+        child: Icon(icon, color: Colors.white, size: 18),
+      ),
+    );
+  }
+}