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/Modal.tsx

@@ -1,35 +1,45 @@
 import { useEffect, useRef, type ReactNode } from 'react'
 
 /**
- * A centered modal dialog rendered over a dimmed backdrop. Closes on Escape or a
- * backdrop click. Wraps content in a native <dialog>-like card; the caller owns
- * the open/closed state (render it only when open).
+ * A centered modal dialog over a dimmed backdrop. Closes on Escape or a backdrop
+ * click, unless `busy` (a mutation is in flight) — then it stays put so the
+ * action can finish and report. The caller owns open/closed state (render only
+ * when open).
  */
 export function Modal({
   title,
   onClose,
+  busy = false,
   children,
 }: {
   title: string
   onClose: () => void
+  busy?: boolean
   children: ReactNode
 }) {
   const cardRef = useRef<HTMLDivElement>(null)
+  // Keep the latest onClose/busy in refs so the mount-only effect below never
+  // re-runs (which would re-attach the listener and steal focus on every parent
+  // re-render, e.g. during a background refetch).
+  const onCloseRef = useRef(onClose)
+  onCloseRef.current = onClose
+  const busyRef = useRef(busy)
+  busyRef.current = busy
 
   useEffect(() => {
+    cardRef.current?.focus()
     function onKey(e: KeyboardEvent) {
-      if (e.key === 'Escape') onClose()
+      if (e.key === 'Escape' && !busyRef.current) onCloseRef.current()
     }
     document.addEventListener('keydown', onKey)
-    cardRef.current?.focus()
     return () => document.removeEventListener('keydown', onKey)
-  }, [onClose])
+  }, [])
 
   return (
     <div
       className="fixed inset-0 z-50 flex items-end justify-center bg-black/40 p-4 sm:items-center"
       onMouseDown={(e) => {
-        if (e.target === e.currentTarget) onClose()
+        if (e.target === e.currentTarget && !busy) onClose()
       }}
     >
       <div