move terminal-verbosity toggle from agent overflow menu to /settings.html

Per mara: 'i wanted you to put this in .../settings.html' — the toggle
belongs with the other operator-local browser preferences, not buried
in each agent's own overflow menu.

Extracted the get/set + localStorage key into @hive/shared/prefs.js so
settings.html (writer) and every per-agent app.js (reader, via
HiveTerminal.create's expandDetails option) agree on the exact same key
without two independently-typed copies that could drift. Removed the
now-unused overflow-menu toggle + its agent.css rules from the agent
page. Docs moved from docs/web-ui/agent.md's overflow-button section to
docs/web-ui/dashboard.md's S3TT1NGS section, next to the existing
browser-notifications preference.

frontend/packages/dashboard/src/settings.html

@@ -31,6 +31,12 @@
       <button type="button" id="notif-unmute" class="btn btn-notif" hidden>🔔 unmute</button>
       <span id="notif-status" class="meta" hidden></span>
     </div>
+
+    <h3>◇ agent terminal</h3>
+    <p class="meta">applies to every agent's terminal viewed in this browser — whether otherwise-collapsed tool-output panels (long results, Write/Edit diffs, …) default open. rows that already default open (send/ask/answer/recv) are unaffected either way.</p>
+    <div id="verbosity-row" class="notif-row">
+      <button type="button" id="expand-details-toggle" class="btn btn-notif" role="switch"></button>
+    </div>
   </main>
 
   <script type="module" src="/static/settings.js" defer></script>