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/TextField.tsx

@@ -1,20 +1,17 @@
-import { forwardRef, useId, type InputHTMLAttributes } from 'react'
+import { forwardRef, type InputHTMLAttributes } from 'react'
 import { cn } from '@/lib/cn'
+import { fieldControlClass, useFieldId } from './field'
 
 interface TextFieldProps extends InputHTMLAttributes<HTMLInputElement> {
   label: string
   hint?: string
 }
 
-// A labelled text input. text-base (16px) is deliberate: smaller fonts make iOS
-// Safari zoom on focus. The field id falls back to the name, then to a generated
-// id, so the label/hint associations always hold.
 export const TextField = forwardRef<HTMLInputElement, TextFieldProps>(function TextField(
   { label, hint, id, name, className, ...props },
   ref,
 ) {
-  const generatedId = useId()
-  const inputId = id ?? name ?? generatedId
+  const inputId = useFieldId(id, name)
   const hintId = hint ? `${inputId}-hint` : undefined
   return (
     <div className="flex flex-col gap-1.5">
@@ -26,13 +23,7 @@ export const TextField = forwardRef<HTMLInputElement, TextFieldProps>(function T
         id={inputId}
         name={name}
         aria-describedby={hintId}
-        className={cn(
-          'w-full rounded-md border border-border bg-surface px-3 py-2 text-base text-fg',
-          'placeholder:text-muted/70 outline-none transition-colors',
-          'focus:border-accent focus:ring-2 focus:ring-accent/30',
-          'disabled:opacity-60',
-          className,
-        )}
+        className={cn(fieldControlClass, className)}
         {...props}
       />
       {hint && (