refactor(z-index): name the stacking layers as a token scale

Replace ~48 magic z-index values (z-10..z-[9999] + 2 inline zIndex) with named
tailwind zIndex tokens (base/menu/chrome/paneltab/panel/rail/overlay/popover/
tooltip/tuner). Values are UNCHANGED — a rename, not a renumber — so layering is
identical; the names give one source of truth for the stack.

frontend/src/components/Header.tsx

@@ -106,7 +106,7 @@ export default function Header({
     // 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">
+    <div className="pointer-events-none absolute top-3 left-3 right-[10%] z-chrome flex items-center gap-3">
       <ModuleName
         label={t(moduleLabelKey[page])}
         labels={moduleLabels}