Add heading IDs, subscript, and superscript to markdown rendering (#21)

markdown-to-jsx has no plugin hook for new inline/block syntax, but it
does correctly parse ordinary links and exposes a slugify callback for
heading anchors -- both get reused the same way this app's own
@mention/#room-reference highlighting already works: ~sub~/^sup^ are
rewritten to a link before compiling (the "URL" is just a carrier for
meaning the parser was never told about), then re-rendered as
<sub>/<sup> instead of an anchor; a heading's {#custom-id} suffix is
stripped from its own text before compiling, and slugify substitutes
the requested id for the auto-generated one.

Applied everywhere markdown renders (chat messages, file previews, the
Help page), not just chat -- MARKDOWN_OPTIONS became a per-render
createMarkdownOptions() since slugify needs each render's own heading
ids.

Definition lists deliberately left unsupported -- no block-level
equivalent to the link-trick exists, and faking one would mean either
reopening the disableParsingRawHTML XSS mitigation or unreliably
misusing blockquote syntax. Documented on the issue.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/HelpPage.tsx

@@ -1,7 +1,7 @@
-import { useEffect, useState } from 'react'
+import { useEffect, useMemo, useState } from 'react'
 import Markdown from 'markdown-to-jsx'
 import { Link } from 'react-router-dom'
-import { MARKDOWN_OPTIONS } from '../components/MessageContent'
+import { createMarkdownOptions, preprocessMarkdown } from '../components/MessageContent'
 import { TopBar } from '../components/TopBar'
 import './HelpPage.css'
 
@@ -13,6 +13,12 @@ const GUIDE_URL = '/USER_GUIDE.md'
 export function HelpPage() {
   const [content, setContent] = useState<string | null>(null)
   const [error, setError] = useState<string | null>(null)
+  // #21: heading-id/sub/superscript support -- the user guide is exactly
+  // the kind of document that benefits from an explicit anchor override.
+  const markdownPreview = useMemo(
+    () => (content !== null ? preprocessMarkdown(content) : null),
+    [content],
+  )
 
   useEffect(() => {
     let cancelled = false
@@ -44,9 +50,11 @@ export function HelpPage() {
         </div>
         {error && <p className="admin-error">{error}</p>}
         {!error && content === null && <p className="help-loading">Loading…</p>}
-        {!error && content !== null && (
+        {!error && markdownPreview && (
           <div className="message-text help-content">
-            <Markdown options={MARKDOWN_OPTIONS}>{content}</Markdown>
+            <Markdown options={createMarkdownOptions(markdownPreview.headingIds)}>
+              {markdownPreview.text}
+            </Markdown>
           </div>
         )}
       </div>