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/LanguageSwitcher.tsx

@@ -103,7 +103,7 @@ export default function LanguageSwitcher({
         </button>
         {open &&
           createPortal(
-            <div style={{ position: "fixed", left: pos.left, bottom: pos.bottom, zIndex: 40 }}>
+            <div className="z-rail" style={{ position: "fixed", left: pos.left, bottom: pos.bottom }}>
               {menu}
             </div>,
             document.body
@@ -127,7 +127,7 @@ export default function LanguageSwitcher({
         <div
           className={`glass-menu absolute ${
             align === "right" ? "right-0" : "left-0"
-          } mt-1 w-40 rounded-xl p-1.5 z-40 animate-[popIn_0.16s_ease]`}
+          } mt-1 w-40 rounded-xl p-1.5 z-rail animate-[popIn_0.16s_ease]`}
         >
           {LANGUAGES.map((l) => (
             <button