Fix small front-end faults found in the audit

- A failed likes request shows the error in the panel instead of an empty list.
- A failed sign-out says why instead of doing nothing.
- Long toasts, like a copied link, wrap instead of running off narrow screens.
- Keys no longer act on the feed behind the help; ? still toggles it, and focus
  goes into the help and back to where it was.
- Phones reach the help from the account panel; the bar has no room for ?.
- Cards next to the current one are inert, so Tab no longer walks into buttons
  that are off screen.
- Keyboard focus is visible everywhere, and panels use the shared colour and
  radius tokens.

web/index.html

@@ -124,6 +124,7 @@
     </section>
     <button type="button" id="logout-btn">sign out</button>
     <button type="button" id="logout-all-btn" class="link">sign out on all devices</button>
+    <button type="button" id="help-open" class="link">gestures and shortcuts</button>
     <a href="/account/export" download>download my data</a>
     <button type="button" id="delete-open" class="link danger">delete account…</button>
     <form id="delete-form" hidden>