feat(block2): default relays + pick-from-list servers (works out of the box)

Feedback: the blank 'web address' field was unusable — nobody knows what to put,
so the market never worked at first launch.
- SocialSettings ships default public relays (nos.lol, relay.damus.io), applied
  automatically for new users so the market works day one. An explicitly empty
  choice still turns the network off (privacy). Exposure is minimal: offers are
  opt-in and carry only a coarse geohash.
- Sharing setup now shows the servers as a short PICK-LIST (checkboxes, pre-
  selected) instead of a blank field; a community's own server goes under a small
  'add another' escape hatch. Intro reworded — no need to know a community.
- Tests updated (defaults) and pinned offline (setRelayUrls([])) so widget tests
  don't hit the network. 22 green, analyzer clean.

apps/app_seeds/lib/ui/market_screen.dart

@@ -409,6 +409,7 @@ class _ConfigSheet extends StatefulWidget {
 class _ConfigSheetState extends State<_ConfigSheet> {
   final _area = TextEditingController();
   final _servers = TextEditingController();
+  final Set<String> _selectedRelays = {};
   bool _loading = true;
   bool _locationBusy = false;
   String? _locationError;
@@ -421,13 +422,22 @@ class _ConfigSheetState extends State<_ConfigSheet> {
 
   Future<void> _load() async {
     _area.text = await widget.settings.areaGeohash();
-    _servers.text = (await widget.settings.relayUrls()).join('\n');
+    final current = await widget.settings.relayUrls();
+    final presetUrls = SocialSettings.presets.map((p) => p.url).toSet();
+    _selectedRelays
+      ..clear()
+      ..addAll(current.where(presetUrls.contains));
+    // Any relays that aren't presets go in the "add another" field.
+    _servers.text = current.where((u) => !presetUrls.contains(u)).join('\n');
     if (mounted) setState(() => _loading = false);
   }
 
   Future<void> _save() async {
     await widget.settings.setAreaGeohash(_area.text);
-    await widget.settings.setRelayUrls(_servers.text.split('\n'));
+    await widget.settings.setRelayUrls([
+      ..._selectedRelays,
+      ..._servers.text.split('\n'),
+    ]);
     if (mounted) Navigator.of(context).pop(true);
   }
 
@@ -523,9 +533,40 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                       style: const TextStyle(color: Color(0xFFB3261E), fontSize: 12),
                     ),
                   ),
-                const SizedBox(height: 8),
-                // The technical web address lives under progressive disclosure —
-                // most people won't have one, and jargon shouldn't greet them.
+                const SizedBox(height: 16),
+                // Pick from a short list of shared servers — pre-selected so it
+                // works out of the box; no jargon-y blank field to puzzle over.
+                Text(
+                  t.market.serversLabel,
+                  style: const TextStyle(
+                    fontSize: 14,
+                    fontWeight: FontWeight.w600,
+                    color: seedOnSurface,
+                  ),
+                ),
+                Text(
+                  t.market.serversHelp,
+                  style: const TextStyle(color: seedMuted, fontSize: 12, height: 1.3),
+                ),
+                for (final preset in SocialSettings.presets)
+                  CheckboxListTile(
+                    key: Key('market.relay.${preset.name}'),
+                    contentPadding: EdgeInsets.zero,
+                    dense: true,
+                    controlAffinity: ListTileControlAffinity.leading,
+                    value: _selectedRelays.contains(preset.url),
+                    title: Text(preset.name),
+                    subtitle: Text(preset.url,
+                        style: const TextStyle(fontSize: 11, color: seedMuted)),
+                    onChanged: (checked) => setState(() {
+                      if (checked ?? false) {
+                        _selectedRelays.add(preset.url);
+                      } else {
+                        _selectedRelays.remove(preset.url);
+                      }
+                    }),
+                  ),
+                // A rarely-needed escape hatch for a community's own server.
                 Theme(
                   data: Theme.of(context)
                       .copyWith(dividerColor: Colors.transparent),
@@ -535,21 +576,16 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                     childrenPadding: const EdgeInsets.only(bottom: 8),
                     title: Text(
                       t.market.serversAdvanced,
-                      style: const TextStyle(
-                        fontSize: 14,
-                        color: seedOnSurface,
-                      ),
+                      style: const TextStyle(fontSize: 13, color: seedMuted),
                     ),
                     children: [
                       TextField(
                         key: const Key('market.servers'),
                         controller: _servers,
-                        minLines: 2,
-                        maxLines: 4,
-                        decoration: InputDecoration(
-                          labelText: t.market.serversLabel,
-                          helperText: t.market.serversHelp,
-                          helperMaxLines: 4,
+                        minLines: 1,
+                        maxLines: 3,
+                        decoration: const InputDecoration(
+                          hintText: 'wss://…',
                         ),
                       ),
                     ],