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/components/Composer.css

@@ -105,6 +105,34 @@
   cursor: not-allowed;
 }
 
+.composer-format-toggle {
+  width: 36px;
+  height: 36px;
+  flex: none;
+  border-radius: var(--radius);
+  background: transparent;
+  color: var(--ds-muted);
+  border: 1px solid var(--ds-border);
+  font-size: 0.82rem;
+  font-weight: 700;
+  line-height: 1;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  cursor: pointer;
+}
+
+.composer-format-toggle:hover:not(:disabled),
+.composer-format-toggle-open {
+  color: var(--ds-text);
+  border-color: var(--ds-accent);
+}
+
+.composer-format-toggle:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
 .composer-attach-wrap {
   position: relative;
   flex: none;