feat(theme): R10 S2 — re-close the rem system (px-frozen labels/chrome -> rem)

The ~124 px-frozen text-[Npx] labels didn't track the Settings font-scale slider
while their rem-sized neighbours did. Add text-2xs (0.6875rem) / text-3xs
(0.625rem) font-size-ONLY tokens (no paired line-height -> matches the arbitrary
they replace, zero vertical-rhythm shift) and codemod every text-[Npx] to a rem
class (rare 8/9/12/13px -> rem arbitraries).

Also: --hdr-h 68px -> 4.25rem so header content clears the rem-sized pills at
scale 1.3 instead of sliding under; NavSidebar/SidePanel slot widths + their
icon/badge px sizes go rem so the rails keep pace with their (now-scaling)
labels; a w-24/h-[54px] thumb -> h-13.5 to preserve 16:9 at scale. Media
thumbnails, page-width caps and fixed container widths deliberately stay px.

Verified via getComputedStyle at scale 1.06/1.3: text-2xs 11.66->14.3px, header
clearance 72->88px, nav slot 246->302px — all scale proportionally.

frontend/src/components/Playlists.tsx

@@ -573,7 +573,7 @@ export default function Playlists() {
       <PageToolbar>
         <div className="px-4 pt-3 pb-2">
           <div className="flex gap-3 items-start mb-4">
-            <div className="w-24 h-[54px] rounded-lg bg-card overflow-hidden shrink-0">
+            <div className="w-24 h-13.5 rounded-lg bg-card overflow-hidden shrink-0">
               {items[0]?.thumbnail_url && (
                 <img src={items[0].thumbnail_url} alt="" className="w-full h-full object-cover" />
               )}
@@ -681,7 +681,7 @@ export default function Playlists() {
                 {linked && detail.dirty && (
                   <span
                     title={t("playlists.unsyncedHint")}
-                    className="inline-flex items-center gap-1.5 text-[11px] text-accent ml-1"
+                    className="inline-flex items-center gap-1.5 text-2xs text-accent ml-1"
                   >
                     <span className="w-1.5 h-1.5 rounded-full bg-accent" />
                     {t("playlists.unsynced")}
@@ -724,7 +724,7 @@ export default function Playlists() {
                   stored order ALREADY matches the sort produced a warning, no Save button and no
                   visible change, which reads as a malfunction. Now the line says which of the three
                   situations you are in; the disabled grip still explains itself on hover/focus. */}
-              {viewActive && <span className="text-[11px] text-muted/80">{viewStatus}</span>}
+              {viewActive && <span className="text-2xs text-muted/80">{viewStatus}</span>}
               {canEdit && (
                 <div className="ml-auto">
                   <UndoToolbar