Retire the Best Value proximity alert option; move Key sheet copy into a tested model

- Alerts: drop the alert-type picker and the Best Value alert path, so
  proximity alerts always follow the cheapest station. A stored `bestValue`
  selection now falls back to cheapest when loaded.
- Key sheet: the price-rating, savings, station-label and favourite wording
  comes from `KeySheetContent` in Shared/FuelSavings.swift — a Foundation-only
  model with tests that pin the exact meaning of each indicator — and the
  sheet is built from that model rather than inline strings.
- BACKLOG: record the plan to restore Best Value proximity alerts behind one
  shared evaluator (user-location baseline, a single disclosed qualification
  rule, and real/Test alert parity).
- Bump CURRENT_PROJECT_VERSION to 29.

FuelBoard/HowBestValueSheet.swift

@@ -130,7 +130,7 @@ struct HowBestValueRow: View {
     var body: some View {
         Button(action: action) {
             HStack(spacing: 8) {
-                Text("How Best Value works")
+                Text(KeySheetContent.helpTitle)
                 Spacer(minLength: 8)
                 Image(systemName: "chevron.forward")
                     .font(.footnote.weight(.semibold))