fix(block2): back buttons on social screens + clear 'Share my seeds' button

Two UX reports:
- Market/Messages (and Profile/Chat) had no back button — they were opened with
  context.go (replaces the stack). Switched the drawer entries, home Market card,
  offer 'Message' link and inbox taps to context.push, so the AppBar shows a back
  arrow to return.
- The market's upload icon (tooltip-only) was unclear. Replaced it with a labelled
  FloatingActionButton.extended 'Share my seeds' (campaign icon); the config gear
  stays in the AppBar.

Analyzer clean.

apps/app_seeds/lib/ui/home_screen.dart

@@ -87,7 +87,7 @@ class HomeScreen extends StatelessWidget {
                       subtitle: t.home.openMarketSubtitle,
                       tag: marketEnabled ? null : t.common.comingSoon,
                       onTap: marketEnabled
-                          ? () => context.go('/market')
+                          ? () => context.push('/market')
                           : null,
                     ),
                   ],