Hero Phase 2: editor shell + background/overlay + draft/preview/publish

Second phase of the hero canvas editor (see HERO_EDITOR.md).

- new lib/heroLayout.js: shared defaultLayout/buildOverlay/heroBackground/
  parseLayout used by both the portal and the editor (Portal refactored onto it)
- new admin view HeroEditor.jsx at /admin/hero (+ sidebar nav + route):
  - live canvas preview (16:9) rendering the draft via HeroElement
  - background panel: image upload (/admin/uploads, >1MB warning), 3x3 position
    grid, overlay opacity slider — all update the canvas in real time
  - debounced (800ms) auto-save to hero_layout_draft
  - Publish (writes hero_layout + draft), Preview (opens /?preview=1), Revert
- Portal: ?preview=1 renders the draft via the admin settings endpoint, with a
  "showing unpublished draft" banner; normal load renders the published layout

No schema/dep changes. Verified end to end: overlay/position update the canvas,
auto-save writes the draft, publish updates the live portal, preview shows the
draft while the public page shows live. Element drag/properties land in Phase 3.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/src/App.jsx

@@ -23,6 +23,7 @@ import AdminLayout from './routes/admin/AdminLayout.jsx'
 import Dashboard from './routes/admin/views/Dashboard.jsx'
 import PostsAdmin from './routes/admin/views/PostsAdmin.jsx'
 import WikiAdmin from './routes/admin/views/WikiAdmin.jsx'
+import HeroEditor from './routes/admin/views/HeroEditor.jsx'
 import SettingsAdmin from './routes/admin/views/SettingsAdmin.jsx'
 import ActivityAdmin from './routes/admin/views/ActivityAdmin.jsx'
 import UsersAdmin from './routes/admin/views/UsersAdmin.jsx'
@@ -66,6 +67,7 @@ export default function App() {
             <Route index element={<Dashboard />} />
             <Route path="posts" element={<PostsAdmin />} />
             <Route path="wiki" element={<WikiAdmin />} />
+            <Route path="hero" element={<HeroEditor />} />
             <Route path="settings" element={<SettingsAdmin />} />
             <Route path="activity" element={<ActivityAdmin />} />
             <Route path="users" element={<UsersAdmin />} />