Security hardening: prevent stored XSS and render crashes

- MarkdownView: add rehype-sanitize between rehype-raw and rehype-katex
  to strip scripts/event-handlers/javascript: URLs from user-authored
  markdown (was stored XSS, also affected the public /pg/* route);
  keep className on code/span/div so KaTeX and syntax highlighting
  still work. Add rehype-sanitize ^6.0.0 to deps and lockfile.
- MarkdownContent / StandaloneMarkdownPage: parse markdown content via
  parseMarkdownContent() instead of an unguarded JSON.parse, so a single
  corrupt/legacy record no longer white-screens the whole page.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -32,6 +32,7 @@
     "redux": "^5.0.1",
     "rehype-katex": "^7.0.1",
     "rehype-raw": "^7.0.0",
+    "rehype-sanitize": "^6.0.0",
     "remark-gfm": "^4.0.0",
     "remark-math": "^6.0.0",
     "util": "^0.12.5"