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/components/Header.tsx

@@ -103,9 +103,10 @@ export default function Header({
   }
 
   return (
-    // Fixed left = the left zone at its widest: nav card (12 margin + 208 + 12 margin = 232) +
-    // filter sidebar (256) + 16 gap = 504. Constant, so it never shifts on collapse/module change.
-    <div className="pointer-events-none fixed top-3 left-4 right-[10%] z-30 flex items-center gap-3 md:left-[504px]">
+    // Absolutely positioned within the (relative) content column, so its left edge tracks the
+    // actual left zone: it sits just inside the content, reclaiming the space when the nav rail is
+    // slim and/or no filter sidebar is present — no hard-coded offset.
+    <div className="pointer-events-none absolute top-3 left-3 right-[10%] z-30 flex items-center gap-3">
       <ModuleName
         label={t(moduleLabelKey[page])}
         labels={moduleLabels}