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/VideoCard.tsx

@@ -245,12 +245,12 @@ function Thumb({
         <div className="w-full h-full" />
       )}
       {video.duration_seconds != null ? (
-        <span className="absolute bottom-1.5 right-1.5 bg-black/80 text-white text-[11px] font-medium px-1.5 py-0.5 rounded-sm">
+        <span className="absolute bottom-1.5 right-1.5 bg-black/80 text-white text-2xs font-medium px-1.5 py-0.5 rounded-sm">
           {formatDuration(video.duration_seconds)}
         </span>
       ) : video.live_status === "live" || video.live_status === "upcoming" ? (
         <span
-          className={`absolute bottom-1.5 right-1.5 text-[10px] font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded ${
+          className={`absolute bottom-1.5 right-1.5 text-3xs font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded ${
             video.live_status === "live" ? "bg-red-500/90 text-white" : "bg-black/80 text-white"
           }`}
         >
@@ -258,7 +258,7 @@ function Thumb({
         </span>
       ) : null}
       {video.live_status === "was_live" && (
-        <span className="absolute top-1.5 left-1.5 bg-accent text-accent-fg text-[10px] font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded-sm">
+        <span className="absolute top-1.5 left-1.5 bg-accent text-accent-fg text-3xs font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded-sm">
           {t("card.stream")}
         </span>
       )}