Copy: England → UK-wide, include Northern Ireland; migrate user-facing copy into Localizable.strings

- Onboarding feature: 'England-wide prices' → 'UK-wide prices — 8,000+ stations, updated twice a day' (key + value unified)
- Stations empty state: 'covers England, Scotland and Wales' → 'covers the whole UK' (dataset includes Northern Ireland)
- Settings Data source footer: source wording updated (CSV mirror or API), OGL attribution retained
- Empty state, browse button and OGL footer now keys in Localizable.strings so the web copy editor covers them
- APPSTORE.md + BACKLOG.md + code comments: England-wide → UK-wide, NI included

APPSTORE.md

@@ -85,8 +85,9 @@ The two 2026-08-11 blockers are RESOLVED by P0 (live chain):
 > approach it. Always-location is requested only when the user enables
 > alerts. The app also provides a home-screen widget showing the current
 > cheapest station nearby, and a price-history Trends chart for starred
-> stations. All data is UK-wide (Fuel Finder covers England, Scotland, and
-> Wales); users outside the UK see an empty state explaining this.
+> stations. All data is UK-wide (Fuel Finder covers England, Scotland, Wales
+> and Northern Ireland); users outside the UK see an empty state explaining
+> this.
 
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