fix(01-03): use empty string instead of undefined for Select value — prevents controlled/uncontrolled switch

src/components/pet/PetForm.tsx

@@ -213,7 +213,7 @@ export function PetForm({
             control={control}
             render={({ field }) => (
               <Select
-                value={field.value || undefined}
+                value={field.value || ""}
                 onValueChange={(val) => {
                   field.onChange(val);
                   setSelectedSpeciesId(val);
@@ -256,8 +256,8 @@ export function PetForm({
             control={control}
             render={({ field }) => (
               <Select
-                value={field.value ?? undefined}
-                onValueChange={(val) => field.onChange(val ?? null)}
+                value={field.value ?? ""}
+                onValueChange={(val) => field.onChange(val || null)}
                 disabled={!selectedSpeciesId}
               >
                 <SelectTrigger