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-lock.json

@@ -10,6 +10,7 @@
       "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",
@@ -1483,6 +1484,19 @@
         "@tiptap/core": "^2.7.0"
       }
     },
+    "node_modules/@tiptap/extension-text-align": {
+      "version": "2.27.2",
+      "resolved": "https://registry.npmjs.org/@tiptap/extension-text-align/-/extension-text-align-2.27.2.tgz",
+      "integrity": "sha512-0Pyks6Hu+Q/+9+5/osoSv0SP6jIerdWMYbi13aaZLsJoj3lBj5WNaE11JtAwSFN5sx0IbqhDSlp1zkvRnzgZ8g==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/ueberdosis"
+      },
+      "peerDependencies": {
+        "@tiptap/core": "^2.7.0"
+      }
+    },
     "node_modules/@tiptap/extension-text-style": {
       "version": "2.27.2",
       "resolved": "https://registry.npmjs.org/@tiptap/extension-text-style/-/extension-text-style-2.27.2.tgz",