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>

USER_GUIDE.md

@@ -70,7 +70,13 @@ send. Use **Shift+Enter** to add a line break without sending.
 
 ## Formatting
 
-Messages support Markdown:
+Messages support Markdown. Prefer buttons to typing the syntax? Click the
+**Aa** button next to the emoji button in the composer to open a formatting
+toolbar — it inserts the markdown below into your message for you (select some
+text first to wrap it, or click a button again to take the formatting back
+off). It stays collapsed until you open it, and remembers your choice.
+
+Or type it yourself:
 
 - `**bold**`, `*italic*`, `~~strikethrough~~`
 - `` `inline code` `` and fenced code blocks (three backticks)