update tooltips

add missing tooltip:
- channels, add channel button
- map, filter nodes button

lib/screens/channels_screen.dart

@@ -299,6 +299,7 @@ class _ChannelsScreenState extends State<ChannelsScreen>
         ),
         floatingActionButton: FloatingActionButton(
           onPressed: () => _showAddChannelDialog(context),
+          tooltip: context.l10n.channels_addChannel,
           child: const Icon(Icons.add),
         ),
         bottomNavigationBar: SafeArea(

lib/screens/map_screen.dart

@@ -354,6 +354,7 @@ class _MapScreenState extends State<MapScreen> {
             ),
             floatingActionButton: FloatingActionButton(
               onPressed: () => _showFilterDialog(context, settingsService),
+              tooltip: context.l10n.map_filterNodes,
               child: const Icon(Icons.filter_list),
             ),
           ),