Add a collapsible markdown formatting toolbar to the composer (#75)

Markdown support was already complete and documented, but nothing in the
composer pointed at it -- unlike @, # and : (which advertise themselves
when typed), formatting syntax was only discoverable via Help, buried in
the account menu.

The composer stays a plain markdown textarea; a new "Aa" button opens a
toolbar that inserts the syntax for you: bold, italic, strikethrough,
inline code, code block, link, quote, bulleted/numbered lists, H1-H3,
sub/superscript. Collapsed by default, remembered across reloads and
room switches. Clicking a button again removes the formatting (native
undo doesn't track programmatic edits), and bold/italic/sub/strike
toggles are run-length aware so "***x***" is handled correctly. The
selection math is pure functions in lib/markdownFormat.ts, unit-tested.

The "?" opens /help#formatting in a new tab so the draft isn't lost;
HelpPage now scrolls to the hash once the guide has loaded, since the
browser's own hash scroll fired before the async-fetched headings
existed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

frontend/src/pages/HelpPage.tsx

@@ -1,6 +1,6 @@
 import { useEffect, useMemo, useState } from 'react'
 import Markdown from 'markdown-to-jsx'
-import { Link } from 'react-router-dom'
+import { Link, useLocation } from 'react-router-dom'
 import { createMarkdownOptions, preprocessMarkdown } from '../components/MessageContent'
 import { TopBar } from '../components/TopBar'
 import './HelpPage.css'
@@ -20,6 +20,15 @@ export function HelpPage() {
     [content],
   )
 
+  // #75: the composer's "?" links straight to /help#formatting, but the guide
+  // is fetched after the page loads -- the browser's own scroll-to-hash has
+  // already run (and found nothing) by the time the headings exist.
+  const { hash } = useLocation()
+  useEffect(() => {
+    if (!markdownPreview || !hash) return
+    document.getElementById(hash.slice(1))?.scrollIntoView()
+  }, [markdownPreview, hash])
+
   useEffect(() => {
     let cancelled = false
     fetch(GUIDE_URL)