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/VideoEditor.tsx

@@ -405,7 +405,7 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
             ))}
             {/* draggable boundary markers */}
             {boundaries.map((b, bi) => (
-              <div key={bi} className="absolute inset-y-0 -ml-1.5 w-3 flex items-center justify-center z-10" style={{ left: pct(b) }}>
+              <div key={bi} className="absolute inset-y-0 -ml-1.5 w-3 flex items-center justify-center z-base" style={{ left: pct(b) }}>
                 <div
                   className="w-1 h-full bg-white/90 cursor-ew-resize rounded"
                   onPointerDown={(e) => {
@@ -426,13 +426,13 @@ export default function VideoEditor({ job, onClose }: { job: DownloadJob; onClos
               </div>
             ))}
             {/* playhead */}
-            <div className="absolute inset-y-0 w-0.5 bg-white pointer-events-none z-20" style={{ left: pct(current) }} />
+            <div className="absolute inset-y-0 w-0.5 bg-white pointer-events-none z-menu" style={{ left: pct(current) }} />
           </div>
 
           {/* hover-scrub thumbnail */}
           {strip && hoverTime != null && (
             <div
-              className="absolute bottom-full mb-2 pointer-events-none z-30 rounded-md overflow-hidden border border-border shadow-xl bg-black"
+              className="absolute bottom-full mb-2 pointer-events-none z-chrome rounded-md overflow-hidden border border-border shadow-xl bg-black"
               style={{ left: hoverLeft, width: HOVER_W }}
             >
               <div style={{ width: HOVER_W, height: HOVER_W / aspect, ...tileStyle(Math.floor(hoverTime / (strip.interval_s || 1))) }} />