feat(nav): slim hover-expand sidebar, role tooltip, logout button

The rail now sits slim by default and expands to a labelled overlay on hover
or keyboard focus without pushing content; the manual toggle became a pin
(pinned = the old in-flow expanded rail). The header no longer reserves a fixed
504px left offset — it's absolutely positioned inside the content column, so it
tracks the actual left zone and reclaims the dead gap. The role badge is gone;
role now shows via a title tooltip on the username, and a logout quick-button
joins the language/about cluster.

frontend/src/App.tsx

@@ -369,8 +369,9 @@ export default function App() {
   // Collapse state for the two full-height panels (left nav + filter sidebar). Persisted to the
   // user's preferences so it follows the account across devices; a localStorage cache seeds the
   // initial render synchronously so nothing flashes open before the server prefs arrive.
+  // Default unpinned (slim + hover-expand overlay); accounts that explicitly pin keep their choice.
   const [navCollapsed, setNavCollapsedState] = useState<boolean>(() =>
-    Boolean(readAccount(LS.navCollapsed, false))
+    Boolean(readAccount(LS.navCollapsed, true))
   );
   const [filterCollapsed, setFilterCollapsedState] = useState<boolean>(() =>
     Boolean(readAccount(LS.filterCollapsed, false))