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/GardenCard.tsx

@@ -32,14 +32,14 @@ export function GardenCard({
         <button
           type="button"
           onClick={onEdit}
-          className="rounded-md px-2.5 py-1 text-sm font-medium text-muted transition-colors hover:bg-border/50 hover:text-fg"
+          className="rounded-md px-2.5 py-1 text-sm font-medium text-muted outline-none transition-colors hover:bg-border/50 hover:text-fg focus-visible:ring-2 focus-visible:ring-accent/40"
         >
           Edit
         </button>
         <button
           type="button"
           onClick={onDelete}
-          className="rounded-md px-2.5 py-1 text-sm font-medium text-muted transition-colors hover:bg-red-500/10 hover:text-red-600 dark:hover:text-red-400"
+          className="rounded-md px-2.5 py-1 text-sm font-medium text-muted outline-none transition-colors hover:bg-red-500/10 hover:text-red-600 focus-visible:ring-2 focus-visible:ring-red-500/40 dark:hover:text-red-400"
         >
           Delete
         </button>