feat(ads): add a link-formatting help aside to the banner message editor

On the campaign detail page, beside the "Add message" form, a static aside
explains the message markdown: plain text is escaped, `[text](url)` becomes a
link, and only http(s)/root-relative targets are linkified (others show as
plain text). New .form-help (flex row) + .help (muted aside) console styles;
wraps below the form on a narrow viewport.

backend/internal/adminconsole/templates/pages/banner_detail.gohtml

@@ -39,11 +39,20 @@
 </div>
 {{else}}<p class="note">no messages yet</p>{{end}}
 <h3>Add message</h3>
+<div class="form-help">
 <form class="form col" method="post" action="/_gm/banners/{{.ID}}/messages">
 <label>English <input type="text" name="body_en" maxlength="500" required></label>
 <label>Russian <input type="text" name="body_ru" maxlength="500" required></label>
 <div><button type="submit">Add message</button></div>
 </form>
+<aside class="help">
+<h4>Formatting</h4>
+<p>Plain text is shown as-is (any HTML is escaped).</p>
+<p>Add a link with markdown — <code>[visible text](https://example.com)</code> — which renders as the linked <em>visible text</em>.</p>
+<p>Only <code>https://</code>, <code>http://</code> and root-relative <code>/path</code> targets become links; any other scheme (e.g. <code>javascript:</code>, <code>ftp:</code>) is dropped and shown as plain text.</p>
+<p>Keep it short: an over-long line scrolls in the strip. The same formatting applies when editing a message above.</p>
+</aside>
+</div>
 </section>
 {{end}}
 {{- end}}