fix(block2): plain-language sharing setup — no relay/server jargon

The market config leaked network jargon ('community servers / one address per
line'), confusing for the 10-80 audience (violates the human-words rule):
- Lead with a plain intro: what sharing does + a community you trust runs a
  shared meeting point, no company in the middle.
- Bury the technical web address under an 'advanced' expander (most people won't
  have one; 'leave empty if you don't have one yet').
- Human labels: 'Your area' (not 'Area code'), 'Web address' under advanced.

Market tests green.

apps/app_seeds/lib/ui/market_screen.dart

@@ -349,6 +349,11 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                     color: seedTitle,
                   ),
                 ),
+                const SizedBox(height: 8),
+                Text(
+                  t.market.setupIntro,
+                  style: const TextStyle(color: seedMuted, fontSize: 13, height: 1.4),
+                ),
                 const SizedBox(height: 16),
                 TextField(
                   key: const Key('market.area'),
@@ -359,16 +364,36 @@ class _ConfigSheetState extends State<_ConfigSheet> {
                     helperMaxLines: 3,
                   ),
                 ),
-                const SizedBox(height: 16),
-                TextField(
-                  key: const Key('market.servers'),
-                  controller: _servers,
-                  minLines: 2,
-                  maxLines: 4,
-                  decoration: InputDecoration(
-                    labelText: t.market.serversLabel,
-                    helperText: t.market.serversHelp,
-                    helperMaxLines: 3,
+                const SizedBox(height: 8),
+                // The technical web address lives under progressive disclosure —
+                // most people won't have one, and jargon shouldn't greet them.
+                Theme(
+                  data: Theme.of(context)
+                      .copyWith(dividerColor: Colors.transparent),
+                  child: ExpansionTile(
+                    key: const Key('market.serversAdvanced'),
+                    tilePadding: EdgeInsets.zero,
+                    childrenPadding: const EdgeInsets.only(bottom: 8),
+                    title: Text(
+                      t.market.serversAdvanced,
+                      style: const TextStyle(
+                        fontSize: 14,
+                        color: seedOnSurface,
+                      ),
+                    ),
+                    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,
+                        ),
+                      ),
+                    ],
                   ),
                 ),
                 const SizedBox(height: 20),