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/ViewSwitcher.tsx

@@ -119,7 +119,7 @@ export default function ViewSwitcher<T extends string>({
           role="menu"
           aria-label={label}
           onKeyDown={onMenuKeyDown}
-          className="glass-menu absolute right-0 top-full mt-1 z-30 min-w-44 p-1.5 rounded-xl animate-[popIn_0.16s_ease]"
+          className="glass-menu absolute right-0 top-full mt-1 z-chrome min-w-44 p-1.5 rounded-xl animate-[popIn_0.16s_ease]"
         >
           {/* The modes are one exclusive choice, so the radio items need a group to scope their
               checked-ness to — otherwise they read as independent checkable items. */}