Add rich-text alignment controls (left/center/right)

Shared RichTextEditor gains @tiptap/extension-text-align for heading and
paragraph nodes, serializing alignment as inline text-align on the block
node so it round-trips through save/reload. Fixed once at the shared
component so it also flows into the upcoming rich_text and two_column
page blocks.

Server sanitize allowlist now permits `style` on p/h1-h6, constrained by
allowedStyles to text-align (left/right/center/justify) only; all other
CSS properties and values are stripped.

Step 1 of the CMS Page Builder spec.

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

client/package.json

@@ -11,6 +11,7 @@
   "dependencies": {
     "@tiptap/extension-image": "^2.27.2",
     "@tiptap/extension-link": "^2.27.2",
+    "@tiptap/extension-text-align": "^2.27.2",
     "@tiptap/react": "^2.27.2",
     "@tiptap/starter-kit": "^2.27.2",
     "diff": "^5.2.2",