feat(wip): implement trip persistence through a local repository. Include loaded trips in the start page UI

frontend/lib/data/datasources/trip_local_datasource.dart

@@ -0,0 +1,83 @@
+import 'dart:convert';
+
+import 'package:shared_preferences/shared_preferences.dart';
+
+/// Defines the contract for persisting trip payloads locally.
+abstract class TripLocalDataSource {
+	/// Returns all saved trip JSON payloads, newest first.
+	Future<List<Map<String, dynamic>>> loadTrips();
+
+	/// Returns a single trip payload by uuid if present.
+  /// TODO - should directly return Trip?
+	Future<Map<String, dynamic>?> getTrip(String uuid);
+
+	/// Upserts the provided trip payload (also used for editing existing trips).
+	Future<void> upsertTrip(Map<String, dynamic> tripJson);
+
+	/// Removes the trip with the matching uuid.
+	Future<void> deleteTrip(String uuid);
+}
+
+class TripLocalDataSourceImpl implements TripLocalDataSource {
+	TripLocalDataSourceImpl({Future<SharedPreferences>? preferences})
+			: _prefsFuture = preferences ?? SharedPreferences.getInstance();
+
+	static const String _storageKey = 'savedTrips';
+	final Future<SharedPreferences> _prefsFuture;
+
+	@override
+	Future<List<Map<String, dynamic>>> loadTrips() async {
+		final prefs = await _prefsFuture;
+		final stored = prefs.getStringList(_storageKey);
+		if (stored == null) return [];
+		return stored.map(_decodeTrip).toList();
+	}
+
+	@override
+	Future<Map<String, dynamic>?> getTrip(String uuid) async {
+		final trips = await loadTrips();
+		for (final trip in trips) {
+			if (trip['uuid'] == uuid) {
+				return Map<String, dynamic>.from(trip);
+			}
+		}
+		return null;
+	}
+
+	@override
+	Future<void> upsertTrip(Map<String, dynamic> tripJson) async {
+		final uuid = tripJson['uuid'];
+		if (uuid is! String || uuid.isEmpty) {
+			throw ArgumentError('Trip JSON must contain a uuid string');
+		}
+
+		final trips = await loadTrips();
+		trips.removeWhere((trip) => trip['uuid'] == uuid);
+		trips.insert(0, Map<String, dynamic>.from(tripJson));
+		await _persistTrips(trips);
+	}
+
+	@override
+	Future<void> deleteTrip(String uuid) async {
+		final trips = await loadTrips();
+		final updated = trips.where((trip) => trip['uuid'] != uuid).toList();
+		if (updated.length == trips.length) {
+			return;
+		}
+		await _persistTrips(updated);
+	}
+
+	Future<void> _persistTrips(List<Map<String, dynamic>> trips) async {
+		final prefs = await _prefsFuture;
+		final payload = trips.map(jsonEncode).toList();
+		await prefs.setStringList(_storageKey, payload);
+	}
+
+	Map<String, dynamic> _decodeTrip(String raw) {
+		final decoded = jsonDecode(raw);
+		if (decoded is! Map<String, dynamic>) {
+			throw const FormatException('Saved trip entry is not a JSON object');
+		}
+		return Map<String, dynamic>.from(decoded);
+	}
+}