Combine scan+receipts into one flow, add plausibility check,

photo viewing, and record drill-down

- Overview is now the single front page (list + "+" FAB to scan)
  instead of a separate near-empty Scan tab; export moved from
  Overview into the drawer
- Drop petrol_station/transaction_number from storage, the edit
  form, and CSV/ODS export - not useful for a personal fuel-cost log
  (destructive Room migration, version 1->2 already shipped earlier)
- Add a live plausibility check (total_price vs. price_per_litre *
  litres - discount) shown as a green/red badge on the review screen
- Add a shared Details/Photo review screen (ReceiptReviewScreen),
  with pinch-to-zoom on the photo (new Coil dependency), reused both
  right after a scan and when drilling into an already-saved receipt
  from the list (ReceiptDetailScreen, with Update/Delete)
- Fix receipt photos not surviving an app restart: camera/gallery
  captures now write to internal storage (filesDir) instead of the
  cache dir, which the OS can clear independent of the database; the
  gallery picker was also saving the Photo Picker's own short-lived
  content:// Uri instead of a Uri onto our persisted copy
- App log: add a Copy button and selectable text
- Fix the connection status bar overlapping the system gesture nav
  bar under edge-to-edge
- Settings: prefill an editable example server URL, add a "Test
  connection" button; both it and the connection status bar now
  surface the actual failure reason, not just pass/fail

android/app/build.gradle.kts

@@ -75,5 +75,6 @@ dependencies {
     implementation(libs.kotlinx.serialization.json)
     implementation(libs.kotlinx.coroutines.android)
     implementation(libs.okhttp)
+    implementation(libs.coil.compose)
     debugImplementation(libs.androidx.ui.tooling)
 }