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  "showing unpublished draft" banner; normal load renders the published layout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/routes/admin/AdminLayout.jsx

@@ -8,6 +8,7 @@ const NAV = [
   { to: '/admin', label: 'Dashboard', end: true },
   { to: '/admin/posts', label: 'Posts' },
   { to: '/admin/wiki', label: 'Wiki' },
+  { to: '/admin/hero', label: 'Hero Editor' },
   { to: '/admin/settings', label: 'Settings' },
   { to: '/admin/activity', label: 'Activity' },
   { to: '/admin/users', label: 'Users' },
@@ -17,6 +18,7 @@ const TITLES = {
   '/admin': 'Dashboard',
   '/admin/posts': 'Posts',
   '/admin/wiki': 'Wiki Pages',
+  '/admin/hero': 'Hero Editor',
   '/admin/settings': 'Site Settings',
   '/admin/activity': 'Activity Log',
   '/admin/users': 'Users',