Make station price age informational only

The relay's price_updated is the maximum price_last_updated across the grades it maps, so it cannot vouch for the selected grade: stop using it to score, gate or warn about Best Value.

- FuelPriceFreshness is now station-level and display-only: one age per station, fresh/outdated/unknown/invalid, "May be outdated" past 30 days, with future-tolerance and non-finite guards.
- Best Value restores the closest-station-selling-the-selected-fuel baseline and its savings ranking; eligibility gating, eligible-first sorting and the unavailable banner are gone.
- Price age appears in the More info sheet only - no listing-row warnings, no extra banners.
- Copy reverted from "closest eligible station" to "closest station"; stale price-confidence strings removed, "May be outdated" added.
- Dormant per-grade timestamp plumbing removed, and regression tests added for every fuel type, missing/invalid/future stamps and the bundled offline dump.

FuelBoardTests/Tests/FuelBoardSharedTests/FuelSavingsCalculationAuditTests.swift

@@ -533,6 +533,6 @@ final class FuelSavingsCalculationAuditTests: XCTestCase {
         )?.result
         XCTAssertEqual(result?.status, .baseline)
         XCTAssertEqual(result?.netSavingGBP, 0)
-        XCTAssertEqual(result?.headline(), "Closest eligible station")
+        XCTAssertEqual(result?.headline(), "Closest station")
     }
 }