Fix apostrophe rendering: prevent HTML entities from matching as tags

The TAG_RE regex in linkifyTags() was matching #39 inside ' as a
tag, breaking the HTML entity and preventing apostrophe rendering.
Added (?<!&) negative lookbehind so # preceded by & is skipped.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/utils/tags.ts

@@ -1,5 +1,5 @@
 const CODE_FENCE_RE = /```[\s\S]*?```|`[^`\n]+`/g;
-const TAG_RE = /(?<!\w)#([\w]+(?:\/[\w]+)*)/g;
+const TAG_RE = /(?<!\w)(?<!&)#([\w]+(?:\/[\w]+)*)/g;
 
 function escapeHtmlAttr(s: string): string {
   return s.replace(/&/g, "&amp;").replace(/"/g, "&quot;").replace(/</g, "&lt;").replace(/>/g, "&gt;");