fix(block2): sharing sheet overflow + location button feedback/robustness

Reported: sheet overflowed 7.7px with the keyboard up, and 'use my location'
seemed to do nothing.
- Config sheet is now scrollable (SingleChildScrollView) — no keyboard overflow.
- 'Use my location' shows an inline spinner + inline error (the old snackbar was
  hidden BEHIND the bottom sheet, so failures were invisible). Message is now
  actionable ('check location is on and the permission is granted').
- Provider hardening: request permission first; on permanent denial open app
  settings; if location services are off open location settings; time-limit the
  fix and fall back to last-known position so it doesn't hang indoors.

Analyzer clean; market tests green.

apps/app_seeds/lib/i18n/en.i18n.json

@@ -357,7 +357,7 @@
     "sharedCount": "Shared {n} seeds nearby",
     "nothingToShare": "Mark some seeds to give, swap or sell first",
     "useLocation": "Use my approximate location",
-    "locationFailed": "Couldn't get your location",
+    "locationFailed": "Couldn't get your location — check that location is on and the permission is granted",
     "queued": "Saved — we'll share these when you're connected"
   },
   "chat": {