Add a real replacement Posts management page (v2.5.0)

Posts was previously just dark-themed skinning over the native
edit.php list table, unlike Comments/Plugins which are genuine
replacements. Built the same way: new PostsData.php REST controller
(list with status/search filters matching native tabs, trash, restore,
permanent delete) and a Vue page matching the Comments layout.

- SubApps.php registers it as aurora-admin-posts, parented under
  edit.php (same pattern as Plugins under plugins.php).
- redirect_native_pages() redirects direct edit.php visits (typed URL,
  not just the sidebar link) to the Aurora page — gated to the default
  post type only, so Pages and other custom post types are untouched.
- MenuSerializer rewrites the sidebar's Posts/All Posts links the same
  way Plugins/Comments already were.
- Gated by Settings > Posts' "Modern posts table" toggle, re-added now
  that it has a real implementation (it was removed a few commits ago
  for describing a feature that didn't exist).
- Fixed modern_plugins/modern_comments/modern_posts all defaulting to
  visually "on" in the Settings UI to match their actual PHP default.

Verified live: trash/restore/status filters/search all work correctly,
edit.php redirects to the Aurora page, edit.php?post_type=page (Pages)
correctly does not.

Co-Authored-By: Claude Sonnet 5 <[email protected]>

app/src/entries/posts.js

@@ -0,0 +1,10 @@
+import { createApp } from 'vue';
+import '@/shell/shell-frame.css';
+import Posts from '@/pages/posts/Posts.vue';
+
+const mountEl = document.getElementById('aurora-admin-posts-root');
+
+if (mountEl) {
+  const data = JSON.parse(mountEl.dataset.auroraAdmin || '{}');
+  createApp(Posts, { serverData: data }).mount(mountEl);
+}