Add on-device debug logging: file capture + share (for real-session testing)

- fileLogger: append redacted API requests/responses + app events to an on-device
  log file (persists across restarts); the client's logSink now writes to it
- Diagnostics screen (Account > Diagnostics): toggle recording, Share log file
  (expo-sharing), clear. StartSession logs the session attempt/result explicitly.
- Lets a real parking session be captured while away from a computer, then shared.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/package.json

@@ -13,6 +13,8 @@
   "dependencies": {
     "parksmarter-client": "*",
     "expo": "~53.0.0",
+    "expo-file-system": "~18.1.11",
+    "expo-sharing": "~13.1.5",
     "expo-secure-store": "~14.0.0",
     "expo-location": "~18.0.0",
     "expo-notifications": "~0.29.0",