Add gardens UI: list page + create/edit/delete (#8)

The /gardens page is now home base, backed by the #7 API.

- lib/units.ts: cm <-> meters/feet conversion + formatCm/formatDimensions
  (metric "m", imperial "ft/in"). Minimal for #8; #9's geometry lib
  consolidates.
- lib/gardens.ts: zod gardenSchema + useGardens/useCreateGarden/
  useUpdateGarden/useDeleteGarden (mutations invalidate the list) and
  conflictGarden() to pull the fresh row out of a 409.
- GardensPage: loading/empty/error states; empty state prompts a first
  garden; responsive card grid (single column on phones).
- GardenCard: name, dimensions formatted per the garden's unit, notes
  preview; body links into /gardens/:id, edit/delete kept out of the link.
- GardenFormModal: create/edit with a unit selector; dimensions are entered
  in the chosen unit and converted to cm (switching units converts the
  current values). A 409 rebases the form onto the server's fresh row.
- DeleteGardenModal: confirmation before removing.
- ui/: Modal (Escape/backdrop close), TextArea, Select.

Verified in a real browser against the embedded binary: create appears
without reload; edit persists (version 2), form prefilled from stored cm
converted back to the garden's unit; delete confirms then removes -> empty
state; an imperial 4 ft x 8 ft garden stores 122 x 244 cm and shows
"4' 0" x 8' 0"". tsc --noEmit 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

@@ -0,0 +1,48 @@
+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).
+ */
+export function Modal({
+  title,
+  onClose,
+  children,
+}: {
+  title: string
+  onClose: () => void
+  children: ReactNode
+}) {
+  const cardRef = useRef<HTMLDivElement>(null)
+
+  useEffect(() => {
+    function onKey(e: KeyboardEvent) {
+      if (e.key === 'Escape') onClose()
+    }
+    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()
+      }}
+    >
+      <div
+        ref={cardRef}
+        role="dialog"
+        aria-modal="true"
+        aria-label={title}
+        tabIndex={-1}
+        className="w-full max-w-md rounded-xl border border-border bg-surface p-6 shadow-lg outline-none"
+      >
+        <h2 className="text-lg font-semibold tracking-tight text-fg">{title}</h2>
+        <div className="mt-4">{children}</div>
+      </div>
+    </div>
+  )
+}