fix(ui): pin edit-sheet Save button as a fixed footer

The variety edit sheet grew with v8 fields (organic, needs-reproduction,
crop calendar) and its Save button, sitting at the bottom of the
SingleChildScrollView, dropped below the fold — the species-linking
widget test could not reach it (tap landed off-screen, species never
linked). Move the fields into a Flexible scroll area and pin the
Cancel/Save row as a fixed footer so Save is always visible and tappable.

Full suite green (233 tests).

apps/app_seeds/lib/ui/variety_detail_screen.dart

@@ -667,6 +667,13 @@ class _EditVarietySheetState extends State<_EditVarietySheet> {
         top: 16,
         bottom: MediaQuery.of(context).viewInsets.bottom + 16,
       ),
+      // Fields scroll; the Cancel/Save row is pinned as a fixed footer so Save
+      // stays visible and tappable no matter how tall the sheet grows.
+      child: Column(
+        mainAxisSize: MainAxisSize.min,
+        crossAxisAlignment: CrossAxisAlignment.stretch,
+        children: [
+          Flexible(
             child: SingleChildScrollView(
               child: Column(
                 mainAxisSize: MainAxisSize.min,
@@ -707,7 +714,9 @@ class _EditVarietySheetState extends State<_EditVarietySheet> {
                       dense: true,
                       key: Key('species.option.${match.id}'),
                       title: Text(match.displayLabel),
-                subtitle: match.family == null ? null : Text(match.family!),
+                      subtitle: match.family == null
+                          ? null
+                          : Text(match.family!),
                       onTap: () => _pick(match),
                     ),
                   const SizedBox(height: 12),
@@ -785,10 +794,15 @@ class _EditVarietySheetState extends State<_EditVarietySheet> {
                       _MonthMultiSelect(
                         label: t.cropCalendar.seedHarvest,
                         mask: _seedHarvestMonths,
-                  onChanged: (m) => setState(() => _seedHarvestMonths = m),
+                        onChanged: (m) =>
+                            setState(() => _seedHarvestMonths = m),
                       ),
                     ],
                   ),
+                ],
+              ),
+            ),
+          ),
           const SizedBox(height: 16),
           Row(
             children: [
@@ -806,7 +820,6 @@ class _EditVarietySheetState extends State<_EditVarietySheet> {
           ),
         ],
       ),
-      ),
     );
   }
 }