Consumer chain: GitHub-only — relay removed from app+widget, Local Network permission gone

The LAN relay is unreachable from any phone outside the developer's LAN, so
for App Store users a relay fallback was dead weight AND harmful: attempting
the private IP fires the iOS Local Network prompt on a stranger's phone and
adds a timeout before the bundled dump. GitHub raw is reachable anywhere the
relay would be, and more reliably.

- LiveChainProvider: GitHub mirror first EVERYWHERE (focused alert path no
  longer relay-first); LAN relay only joins behind the hidden dev flag
  fuelboard.relayFallback (Settings → Debug, keychain-first, off by default)
- MirrorFuelProvider: focused() projection replaces the relay's server-side
  radius endpoint (fuel filter + within-radius + nearest-first + limit 500)
  — the alert path re-checks radius/fuel itself, so behaviour is identical
- Widget: fetchFocused/fetchFuelOnly now GitHub-first via the chain (day-cache
  shared through the app group); relay + widget-diag GET gated on the same
  dev flag — off-LAN widgets no longer attempt local network at all
- FuelBeacon: fires only when the dev flag is on — consumers make zero
  local-network attempts; /stats app-hit spike = GitHub down + dev flag on
- Onboarding: Local Network machinery removed entirely (no probe, no prompt,
  no 12 s stall) — data page is now informational: prices download from the
  internet, no permissions needed. Replay on cellular can no longer hang.
- Info.plist: NSLocalNetworkUsageDescription copy → optional dev-only wording
- 95 tests (6 new: relay-skip-when-disabled x2, focused prefer-mirror, mirror
  focused projection x3, beacon dev-flag gate)

Config/App-Info.plist

@@ -38,7 +38,7 @@
 	<key>NSLocationAlwaysAndWhenInUseUsageDescription</key>
 	<string>FuelBoard uses Always location to alert you when you approach the cheapest station nearby.</string>
 	<key>NSLocalNetworkUsageDescription</key>
-	<string>FuelBoard connects to the FuelBoard Relay on your local network to download the latest fuel prices.</string>
+	<string>FuelBoard can optionally connect to the FuelBoard Relay on your local network for faster alerts during development.</string>
 	<key>NSSupportsLiveActivities</key>
 	<true/>
 	<key>UIBackgroundModes</key>