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/PlexSidebar.tsx

@@ -46,11 +46,11 @@ function MatchToggle({
   const { t } = useTranslation();
   return (
     <div className="mb-1.5 flex items-center justify-between">
-      <span className="text-[10px] uppercase tracking-wide text-muted">
+      <span className="text-3xs uppercase tracking-wide text-muted">
         {t("plex.filter.matchLabel")}
       </span>
       <div
-        className="flex items-center rounded-full border border-border bg-card p-0.5 text-[10px]"
+        className="flex items-center rounded-full border border-border bg-card p-0.5 text-3xs"
         role="group"
         aria-label={t("plex.filter.matchLabel")}
       >