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/pages/GardensPage.tsx

@@ -12,14 +12,14 @@ type Dialog = { kind: 'create' } | { kind: 'edit'; garden: Garden } | { kind: 'd
 export function GardensPage() {
   const gardens = useGardens()
   const [dialog, setDialog] = useState<Dialog>(null)
+  const close = () => setDialog(null)
+  const hasGardens = !!gardens.data && gardens.data.length > 0
 
   return (
     <section>
       <div className="flex items-center justify-between gap-4">
         <h1 className="text-2xl font-semibold tracking-tight">Gardens</h1>
-        {gardens.data && gardens.data.length > 0 && (
-          <Button onClick={() => setDialog({ kind: 'create' })}>New garden</Button>
-        )}
+        {hasGardens && <Button onClick={() => setDialog({ kind: 'create' })}>New garden</Button>}
       </div>
 
       <div className="mt-6">
@@ -50,9 +50,9 @@ export function GardensPage() {
         )}
       </div>
 
-      {dialog?.kind === 'create' && <GardenFormModal onClose={() => setDialog(null)} />}
-      {dialog?.kind === 'edit' && <GardenFormModal garden={dialog.garden} onClose={() => setDialog(null)} />}
-      {dialog?.kind === 'delete' && <DeleteGardenModal garden={dialog.garden} onClose={() => setDialog(null)} />}
+      {dialog?.kind === 'create' && <GardenFormModal onClose={close} />}
+      {dialog?.kind === 'edit' && <GardenFormModal garden={dialog.garden} onClose={close} />}
+      {dialog?.kind === 'delete' && <DeleteGardenModal garden={dialog.garden} onClose={close} />}
     </section>
   )
 }