fix(mobile): make Bluetooth survive iOS Guided Access (kiosk lock)

[burkben]

Bug report

Using the app, the speedometer registered nothing. Turned out Guided Access was on. Turning Guided Access off made it work again — but then I was completely locked out of starting Guided Access, with iOS showing an error.

Root cause

All three symptoms trace to one iOS conflict: CoreBluetooth's system Bluetooth dialog vs. Guided Access.

• iOS suppresses system dialogs while a Guided Access session is active, and conversely a pending system Bluetooth dialog blocks Guided Access from starting.
• react-native-ble-plx shows iOS's system Bluetooth permission/power alert when BT is off or not-yet-authorized. That single modal explains everything:
  1. Scanning under Guided Access → the system BT alert is suppressed → portal never connects, gauge is dead.
  2. Turning Guided Access off → the alert can finally resolve → works again.
  3. The lingering system BT alert → blocks Guided Access from starting → the iOS error.

react-native-ble-plx@3.5.1 exposes no JS option to suppress that alert (CBCentralManagerOptionShowPowerAlertKey isn't surfaced), so this is fixed in-app + docs, with no native patch.

Changes

Mobile

• ble/bleStatus.ts (+ unit tests) — pure BlePhase → banner mapping for poweredOff / unauthorized / unsupported / error.
• components/BleStatusBanner.tsx — renders that banner on the home speedometer screen, with an Open Settings shortcut for the permission case. Previously these states only showed on the Live-log screen, so under Guided Access the gauge sat silently dead with no explanation.
• ble/blePortal.ts → prewarmBle(), called from app/_layout.tsx at launch on a physical device, so the iOS Bluetooth permission/power prompts fire on the home screen — not mid-session inside Guided Access.

Docs

• New docs/guides/ios-guided-access.md — kiosk setup (turn BT on + connect once before locking down) and troubleshooting for both the dead gauge and the "Guided Access could not be started" error.
• Linked from README.md and docs/guides/ios-testflight.md.

Trade-off

The launch-time prewarm means a one-time Bluetooth permission prompt (and a "Bluetooth is off" alert, if applicable) now appears at app startup on a real device — intentional, to keep those prompts out of Guided Access sessions.

Validation

• npm run typecheck — clean across workspaces.
• npm test — 186 mobile + 49 protocol tests pass, including the new bleStatus tests.
• No device build performed (CI-only environment); BLE/Guided-Access behavior is documented for on-device verification.

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