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/seed_glyph.dart

@@ -1,5 +1,6 @@
 import 'package:commons_core/commons_core.dart';
 import 'package:flutter/material.dart';
+import 'package:material_symbols_icons/symbols.dart';
 
 /// Renders a glyph from the bundled `seedks` icon font, whose characters `a`–`k`
 /// map to seed-themed pictograms (jar, packet, spoon…). See
@@ -63,7 +64,7 @@ IconData _materialIconForKind(QuantityKind kind) => switch (kind) {
   QuantityKind.cob || QuantityKind.ear => Icons.grass,
   QuantityKind.head || QuantityKind.seedHead => Icons.filter_vintage_outlined,
   QuantityKind.fruit => Icons.local_florist_outlined,
-  QuantityKind.plant => Icons.local_florist_outlined,
+  QuantityKind.plant => Symbols.potted_plant,
   QuantityKind.pot => Icons.yard_outlined,
   QuantityKind.tray => Icons.grid_on_outlined,
   QuantityKind.bulb ||