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/ui/field.ts

@@ -0,0 +1,18 @@
+import { useId } from 'react'
+
+// Shared field plumbing for the labelled form controls (TextField, TextArea,
+// Select) so their id-fallback and base styling stay in one place.
+
+/** The field id: an explicit id, else the name, else a generated stable id, so
+ *  the label's htmlFor always binds to something. */
+export function useFieldId(id?: string, name?: string): string {
+  const generated = useId()
+  return id ?? name ?? generated
+}
+
+/** Base classes shared by the text/select/textarea controls. text-base (16px)
+ *  keeps iOS Safari from zooming on focus. */
+export const fieldControlClass =
+  'w-full rounded-md border border-border bg-surface px-3 py-2 text-base text-fg ' +
+  'outline-none transition-colors placeholder:text-muted/70 ' +
+  'focus:border-accent focus:ring-2 focus:ring-accent/30 disabled:opacity-60'