Replace the mobile tab bar with a hamburger menu

Six tabs (Queue, On-call, Alerts, Team, Admin, Account) had already
outgrown the bottom bar once: 56b8191 let it auto-size its columns to
fit however many there were, but on a phone that only left each tab
55-65px wide. Squeezing further wasn't an option, so the bar is gone
on mobile and a hamburger button in the topbar opens a menu instead.

The menu reuses the sheet + menu-item pattern already used for the
snooze and assign-to actions in incident.js, rather than a new overlay
component. It lists the same sections the sidebar does, including
hiding Admin for non-admins, and the triggered-incident badge that
used to sit on the Queue tab icon now shows on the hamburger button.

Desktop (>=900px) is untouched: the sidebar is the same markup, CSS
alone hides it below 900px and shows it above, so nothing there
changed behaviourally.

Claude-Session: https://claude.ai/code/session_01RHPj4ggeFdEjKKfm4SHbD7

internal/web/static/index.html

@@ -108,7 +108,13 @@
     </nav>
 
     <header class="topbar">
-      <h1 class="topbar-title" id="topbar-title">Queue</h1>
+      <div class="topbar-left">
+        <button class="btn btn-ghost btn-icon" id="menu-btn" type="button" aria-label="Menu" aria-haspopup="menu">
+          <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M4 7h16M4 12h16M4 17h16"/></svg>
+          <span class="nav-badge menu-btn-badge" data-badge hidden></span>
+        </button>
+        <h1 class="topbar-title" id="topbar-title">Queue</h1>
+      </div>
       <span class="open-pill" id="open-pill" hidden></span>
     </header>