Added report functionality and Dropbox connection

lib/services/fuel_report.dart

@@ -0,0 +1,81 @@
+import '../models/fuel_entry.dart';
+import '../models/vehicle.dart';
+
+/// One vehicle's totals within a [FuelReport]'s date range.
+class VehicleReportRow {
+  final Vehicle vehicle;
+  final int entryCount;
+  final double totalGallons;
+  final double totalCost;
+
+  VehicleReportRow({
+    required this.vehicle,
+    required this.entryCount,
+    required this.totalGallons,
+    required this.totalCost,
+  });
+}
+
+/// Per-vehicle fuel totals for entries with a receipt, dated within
+/// [startDate]–[endDate] inclusive. See [buildFuelReport].
+class FuelReport {
+  final DateTime startDate;
+  final DateTime endDate;
+  final List<VehicleReportRow> rows;
+
+  FuelReport({required this.startDate, required this.endDate, required this.rows});
+
+  double get totalGallons => rows.fold(0.0, (sum, r) => sum + r.totalGallons);
+  double get totalCost => rows.fold(0.0, (sum, r) => sum + r.totalCost);
+}
+
+/// Builds a per-vehicle fuel summary for [startDate]–[endDate] (inclusive,
+/// judged by each entry's purchase date, in local time), restricted to
+/// entries that actually have a receipt attached — a report meant to
+/// substantiate a fuel tax refund claim is only as good as the
+/// documentation behind each entry, so an entry logged without a photo
+/// isn't something the report should be claiming on your behalf. "Has a
+/// receipt" means a local copy or a cloud copy — [FuelEntry.receiptImagePath]
+/// or [FuelEntry.receiptDriveFileId] — not specifically an already-synced
+/// one, since sync status is a technical detail unrelated to whether the
+/// purchase is documented.
+///
+/// Vehicles with no qualifying entries in range are omitted rather than
+/// shown with an all-zero row. Rows are sorted by [Vehicle.displayLabel].
+FuelReport buildFuelReport({
+  required List<Vehicle> vehicles,
+  required List<FuelEntry> fuelEntries,
+  required DateTime startDate,
+  required DateTime endDate,
+}) {
+  final rangeStart = DateTime(startDate.year, startDate.month, startDate.day);
+  final rangeEndExclusive =
+      DateTime(endDate.year, endDate.month, endDate.day).add(const Duration(days: 1));
+
+  final vehiclesById = {for (final v in vehicles) v.id: v};
+  final entriesByVehicle = <String, List<FuelEntry>>{};
+
+  for (final entry in fuelEntries) {
+    final hasReceipt = entry.receiptImagePath != null || entry.receiptDriveFileId != null;
+    if (!hasReceipt) continue;
+    if (entry.date.isBefore(rangeStart) || !entry.date.isBefore(rangeEndExclusive)) continue;
+    (entriesByVehicle[entry.vehicleId] ??= []).add(entry);
+  }
+
+  final rows = <VehicleReportRow>[];
+  for (final entry in entriesByVehicle.entries) {
+    final vehicle = vehiclesById[entry.key];
+    // No matching (active) vehicle — e.g. deleted since — nothing sane to
+    // attribute these entries to.
+    if (vehicle == null) continue;
+    rows.add(VehicleReportRow(
+      vehicle: vehicle,
+      entryCount: entry.value.length,
+      totalGallons: entry.value.fold(0.0, (sum, e) => sum + e.gallons),
+      totalCost: entry.value.fold(0.0, (sum, e) => sum + e.totalCost),
+    ));
+  }
+  rows.sort((a, b) => a.vehicle.displayLabel.compareTo(b.vehicle.displayLabel));
+
+  return FuelReport(startDate: rangeStart, endDate: endDate, rows: rows);
+}