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/Tooltip.tsx

@@ -83,7 +83,7 @@ export default function Tooltip({
               top: coords.top,
               transform: `translateX(-50%) translateY(${coords.placement === "top" ? "-100%" : "0"})`,
             }}
-            className="glass pointer-events-none z-[100] w-max max-w-[240px] px-2.5 py-1.5 rounded-lg text-xs leading-snug text-fg font-normal normal-case tracking-normal animate-[fadeIn_0.12s_ease]"
+            className="glass pointer-events-none z-tooltip w-max max-w-[240px] px-2.5 py-1.5 rounded-lg text-xs leading-snug text-fg font-normal normal-case tracking-normal animate-[fadeIn_0.12s_ease]"
           >
             {hint}
           </div>,