refine(quantity): drop handful, reorder scale, potted-plant icon, year dropdown

Feedback from the running app:
- Remove "handful" from the scale (redundant with spoon); keep the enum value.
- Reorder so "packet" (sobre) follows "spoon" (cuchara).
- Use the Material Symbols potted-plant for the plant lot type and plant unit
  (adds material_symbols_icons).
- Harvest year is now a dropdown of recent years instead of free text.

38 tests green; Linux build runs with the new icon font bundled.

apps/app_seeds/lib/ui/variety_detail_screen.dart

@@ -477,7 +477,11 @@ class _EditVarietySheetState extends State<_EditVarietySheet> {
 
 Future<void> _showAddLotSheet(BuildContext context, VarietyDetailCubit cubit) {
   final t = context.t;
-  final yearController = TextEditingController();
+  final currentYear = DateTime.now().year;
+  // Harvest years: current year down through the last four decades. Newest
+  // first because a freshly harvested lot is the common case.
+  final years = [for (var y = currentYear; y >= currentYear - 40; y--) y];
+  var selectedYear = currentYear;
   var selectedType = LotType.seed;
   Quantity? selectedQuantity;
   return showModalBottomSheet<void>(
@@ -508,14 +512,20 @@ Future<void> _showAddLotSheet(BuildContext context, VarietyDetailCubit cubit) {
               }),
             ),
             const SizedBox(height: 12),
-            TextField(
+            DropdownButtonFormField<int>(
               key: const Key('addLot.year'),
-              controller: yearController,
-              keyboardType: TextInputType.number,
+              initialValue: selectedYear,
               decoration: InputDecoration(
                 labelText: t.addLot.year,
                 border: const OutlineInputBorder(),
               ),
+              items: [
+                for (final y in years)
+                  DropdownMenuItem(value: y, child: Text('$y')),
+              ],
+              onChanged: (value) {
+                if (value != null) setState(() => selectedYear = value);
+              },
             ),
             const SizedBox(height: 16),
             Text(
@@ -541,7 +551,7 @@ Future<void> _showAddLotSheet(BuildContext context, VarietyDetailCubit cubit) {
                   onPressed: () {
                     cubit.addLot(
                       type: selectedType,
-                      year: int.tryParse(yearController.text.trim()),
+                      year: selectedYear,
                       quantity: selectedQuantity,
                     );
                     Navigator.of(sheetContext).pop();