Make the hero Moon image configurable via props.src

The Moon stays a dedicated, first-class hero element — only its image
source becomes configurable. Adds optional src/alt props alongside the
existing size/glow.

- HeroElement: the moon renders props.src when present, else falls back to
  the default /assets/img/hero-moon.png. Size, glow, and animation are
  unchanged. alt is now props.alt (default '', same as before).
- HeroEditor MoonPanel: adds an image upload (reusing the existing shared
  api.admin.upload workflow, same as the image/background panels) that sets
  props.src, an alt-text field, and a "Use default" reset. Size/glow
  controls unchanged.

Fully backwards compatible: existing layouts with only size/glow and no
src render exactly as today via the fallback. No DB, API, or hero-JSON
changes; no migration.

client/src/components/HeroElement.jsx

@@ -107,12 +107,15 @@ function content(element) {
     case 'buttons':
       return <Buttons props={element.props || {}} />
     case 'moon': {
-      const size = element.props?.size || 96
-      const glow = element.props?.glow ?? 0.45
+      const props = element.props || {}
+      const size = props.size || 96
+      const glow = props.glow ?? 0.45
+      // Image source is configurable; old layouts with no src fall back to the
+      // default hero moon so they render exactly as before. Size/glow unchanged.
       return (
         <img
-          src={MOON_IMAGE}
-          alt=""
+          src={props.src || MOON_IMAGE}
+          alt={props.alt || ''}
           draggable={false}
           style={{
             width: size,

client/src/routes/admin/views/HeroEditor.jsx

@@ -499,9 +499,45 @@ const swatch = { width: '100%', height: 38, padding: 2, border: '1px solid var(-
 
 function MoonPanel({ element, onProps }) {
   const p = element.props || {}
+  const [up, setUp] = useState(false)
+  // Reuses the shared admin upload endpoint (same as the image/background panels);
+  // a successful upload just points props.src at the returned URL.
+  async function onFile(e) {
+    const f = e.target.files?.[0]
+    e.target.value = ''
+    if (!f) return
+    if (f.size > 1024 * 1024 && !confirm('This image is over 1 MB and may slow the page. Upload anyway?')) return
+    setUp(true)
+    try {
+      const { url } = await api.admin.upload(f)
+      onProps({ src: url })
+    } catch {
+      /* ignore */
+    } finally {
+      setUp(false)
+    }
+  }
   return (
     <div style={{ display: 'flex', flexDirection: 'column', gap: 14 }}>
-      <p className="sans dim" style={{ margin: 0, fontSize: '0.8rem' }}>Uses the moon from the hero artwork.</p>
+      <div>
+        <span className="field-label">Moon image</span>
+        {p.src ? (
+          <img src={p.src} alt="" style={{ width: '100%', maxHeight: 90, objectFit: 'contain', borderRadius: 6, border: '1px solid var(--line)', marginBottom: 8 }} />
+        ) : (
+          <p className="sans dim" style={{ margin: '0 0 8px', fontSize: '0.8rem' }}>Using the default moon from the hero artwork.</p>
+        )}
+        <label className="btn btn-ghost btn-sq" style={{ display: 'inline-block', cursor: 'pointer' }}>
+          {up ? 'Uploading…' : p.src ? 'Replace' : 'Upload'}
+          <input type="file" accept="image/*" onChange={onFile} hidden disabled={up} />
+        </label>
+        {p.src && (
+          <span className="link-accent" style={{ fontSize: '0.8rem', marginLeft: 10 }} onClick={() => onProps({ src: '' })}>Use default</span>
+        )}
+      </div>
+      <label>
+        <span className="field-label">Alt text</span>
+        <input className="input" value={p.alt || ''} onChange={(e) => onProps({ alt: e.target.value })} />
+      </label>
       <label>
         <span className="field-label">Size — {p.size || 96}px</span>
         <input type="range" min="24" max="320" step="1" value={p.size || 96} onChange={(e) => onProps({ size: Number(e.target.value) })} style={{ width: '100%' }} />