Add LPG bi-fuel and hydrogen fuel types

Add petrol_lpg, diesel_lpg and hydrogen to the fuel_type choices, across
the six places that define them: the PocketBase schema, the car form and
detail view in both the Web App and the Phone App, and the two doc
comments that enumerate the values. The Go API needed no change — it
passes fuel_type through as a free string, so PocketBase is the only
validator.

Model the LPG conversions as their own choices rather than a separate
"has LPG" flag: the car runs on either tank, so "petrol + LPG" is what an
owner picks it out as. Order each variant next to its base fuel so the
dropdowns read naturally.

Purely additive — existing rows keep their values and need no migration.
The live PocketBase schema does still need scripts/setup-pocketbase.mjs
re-run before the new choices will save, since its select field allows
only the old four; the script reconciles select values on existing
fields, so re-running migrates it in place.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Phone App/lib/models.dart

@@ -18,7 +18,7 @@ class Car {
   final String differentialOilSpec;
   final String brakeFluidSpec;
   final String coolantSpec;
-  final String fuelType; // petrol | diesel | hybrid | electric
+  final String fuelType; // petrol | petrol_lpg | diesel | diesel_lpg | hybrid | electric | hydrogen
   final String buildDate; // ISO YYYY-MM-DD (date-only)
   final String firstRegistrationDate; // ISO YYYY-MM-DD (date-only)
   final int serviceIntervalDays;

Phone App/lib/screens/car_detail_screen.dart

@@ -456,9 +456,12 @@ class _InfoTab extends StatelessWidget {
 
   static const Map<String, String> _fuelLabels = {
     "petrol": "Petrol (gasoline)",
+    "petrol_lpg": "Petrol (gasoline) + LPG",
     "diesel": "Diesel",
+    "diesel_lpg": "Diesel + LPG",
     "hybrid": "Hybrid",
     "electric": "Electric",
+    "hydrogen": "Hydrogen",
   };
   static String _fuelLabel(String v) => _fuelLabels[v] ?? "—";
 

Phone App/lib/screens/car_form_sheet.dart

@@ -198,9 +198,12 @@ class _CarFormSheetState extends State<CarFormSheet> {
                     labelText: "Fuel type", border: OutlineInputBorder(), isDense: true),
                 items: const [
                   DropdownMenuItem(value: "petrol", child: Text("Petrol (gasoline)")),
+                  DropdownMenuItem(value: "petrol_lpg", child: Text("Petrol (gasoline) + LPG")),
                   DropdownMenuItem(value: "diesel", child: Text("Diesel")),
+                  DropdownMenuItem(value: "diesel_lpg", child: Text("Diesel + LPG")),
                   DropdownMenuItem(value: "hybrid", child: Text("Hybrid")),
                   DropdownMenuItem(value: "electric", child: Text("Electric")),
+                  DropdownMenuItem(value: "hydrogen", child: Text("Hydrogen")),
                 ],
                 onChanged: (v) => setState(() => _fuelType = v ?? ""),
               ),