Address Gadfly review on #8: modal focus/close, dim validation, dedup

Fixes from the PR #27 adversarial review (considered; not graded).

Correctness / error-handling
- Modal: focus once on mount and attach the keydown listener once (latest
  onClose/busy via refs), so a parent re-render (e.g. a background refetch)
  no longer re-fires focus() and steals it from the input being typed in.
- Modal takes a `busy` prop; while a save/delete is in flight, Escape and
  backdrop clicks don't dismiss it (form/delete modals pass busy=pending),
  so an action can't be interrupted mid-flight and lose its error.
- Form validates the *converted* centimeter values against the server's
  [1cm, 100m] bounds, so sub-cm / over-100m sizes fail client-side with a
  clear message instead of a generic server error.
- displayFromCm rounds imperial to 0.1 ft so an 8 ft garden (244 cm)
  prefills as "8", not "8.01".

Maintainability
- Extracted ui/field.ts (useFieldId + fieldControlClass) shared by
  TextField/TextArea/Select. Added a `danger` Button variant, used by the
  delete modal; card edit/delete buttons gained focus-visible rings.
- useUpdateGarden takes the id in its mutation variables (no dummy 0 during
  create). GardensPage shares a close handler; dropped a redundant zod
  annotation; 409 rebase reuses dimString; renamed `del` -> `deletion`.

Verified in a browser: 8 ft round-trips to a prefilled "8"; a sub-cm value
is rejected client-side with the modal staying open. tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01JdQpdYYsTgtkJBxbcpAszi

web/src/components/gardens/DeleteGardenModal.tsx

@@ -7,13 +7,13 @@ import { useDeleteGarden, type Garden } from '@/lib/gardens'
 
 /** Confirmation dialog for deleting a garden (and everything in it). */
 export function DeleteGardenModal({ garden, onClose }: { garden: Garden; onClose: () => void }) {
-  const del = useDeleteGarden()
+  const deletion = useDeleteGarden()
   const [error, setError] = useState<string | null>(null)
 
   async function onConfirm() {
     setError(null)
     try {
-      await del.mutateAsync(garden.id)
+      await deletion.mutateAsync(garden.id)
       onClose()
     } catch (err) {
       setError(errorMessage(err, 'Could not delete the garden.'))
@@ -21,7 +21,7 @@ export function DeleteGardenModal({ garden, onClose }: { garden: Garden; onClose
   }
 
   return (
-    <Modal title="Delete garden" onClose={onClose}>
+    <Modal title="Delete garden" onClose={onClose} busy={deletion.isPending}>
       <div className="flex flex-col gap-4">
         <p className="text-sm text-muted">
           Delete <span className="font-medium text-fg">{garden.name}</span> and everything planned in it?
@@ -29,16 +29,11 @@ export function DeleteGardenModal({ garden, onClose }: { garden: Garden; onClose
         </p>
         {error && <Alert>{error}</Alert>}
         <div className="flex justify-end gap-2">
-          <Button type="button" variant="ghost" onClick={onClose} disabled={del.isPending}>
+          <Button type="button" variant="ghost" onClick={onClose} disabled={deletion.isPending}>
             Cancel
           </Button>
-          <Button
-            type="button"
-            onClick={onConfirm}
-            disabled={del.isPending}
-            className="bg-red-600 text-white hover:bg-red-700"
-          >
-            {del.isPending ? 'Deleting…' : 'Delete'}
+          <Button type="button" variant="danger" onClick={onConfirm} disabled={deletion.isPending}>
+            {deletion.isPending ? 'Deleting…' : 'Delete'}
           </Button>
         </div>
       </div>