fix: cap request bodies, link login error, add book-form retry, guard double add

Limit request bodies to 1 MiB (413 JSON), wire aria-describedby on the
login form, add Retry and cancellation to the book form load, disable
Add entry while a note POST is in flight, and poll pg_isready in README.

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

src/books/BookForm.tsx

@@ -19,11 +19,16 @@ export function BookForm() {
   const [loadError, setLoadError] = useState<string | null>(null)
   const [error, setError] = useState<FormError | null>(null)
   const [busy, setBusy] = useState(false)
+  const [attempt, setAttempt] = useState(0)
 
   useEffect(() => {
     if (!editing) return
+    let cancelled = false
+    setLoadError(null)
+    setLoading(true)
     api<Book>('GET', `/books/${id}`).then(
       (book) => {
+        if (cancelled) return
         setFields({
           title: book.title,
           author: book.author,
@@ -34,11 +39,15 @@ export function BookForm() {
         setLoading(false)
       },
       (err) => {
+        if (cancelled) return
         setLoadError(errorMessage(err))
         setLoading(false)
       },
     )
-  }, [editing, id])
+    return () => {
+      cancelled = true
+    }
+  }, [editing, id, attempt])
 
   function set(name: keyof Fields) {
     return (event: { target: { value: string } }) => setFields((prev) => ({ ...prev, [name]: event.target.value }))
@@ -72,13 +81,16 @@ export function BookForm() {
 
   if (loadError) {
     return (
-      <div className={card}>
-        <p role="alert" className="text-red-700">
-          {loadError}
-        </p>
-        <Link to="/books" className="mt-2 inline-block text-indigo-700 underline">
-          ← Back to library
-        </Link>
+      <div role="alert" className={card}>
+        <p className="text-red-700">{loadError}</p>
+        <div className="mt-2 flex items-center gap-3">
+          <button type="button" className={secondaryButton} onClick={() => setAttempt((n) => n + 1)}>
+            Retry
+          </button>
+          <Link to="/books" className="text-indigo-700 underline">
+            ← Back to library
+          </Link>
+        </div>
       </div>
     )
   }
@@ -106,7 +118,7 @@ export function BookForm() {
           ))}
         </select>
       </label>
-      {genresError && <p className="text-sm text-red-700">{`Could not load genres: ${genresError}`}</p>}
+      {genresError && <p role="alert" className="text-sm text-red-700">{`Could not load genres: ${genresError}`}</p>}
       <div className="flex gap-4">
         <label className="block">
           <span className={labelText}>Total pages</span>