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/PlayerModal.tsx

@@ -1056,7 +1056,7 @@ export default function PlayerModal({
             )}
             {/* Discreet badge while YouTube's own controls have taken over (overlay yielded). */}
             {playerError == null && nativeControls && (
-              <div className="pointer-events-none absolute top-3 left-3 z-menu flex items-center gap-1.5 rounded-full bg-black/60 px-2.5 py-1 text-[11px] text-white/90 shadow-lg backdrop-blur-xs">
+              <div className="pointer-events-none absolute top-3 left-3 z-menu flex items-center gap-1.5 rounded-full bg-black/60 px-2.5 py-1 text-2xs text-white/90 shadow-lg backdrop-blur-xs">
                 <Settings className="h-3.5 w-3.5" />
                 {t("player.nativeControls")}
               </div>
@@ -1250,7 +1250,7 @@ export default function PlayerModal({
                     <span>· {formatDuration(active.duration_seconds)}</span>
                   )}
                   {active.live_status === "was_live" && (
-                    <span className="bg-accent text-accent-fg text-[10px] font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded-sm">
+                    <span className="bg-accent text-accent-fg text-3xs font-semibold uppercase tracking-wide px-1.5 py-0.5 rounded-sm">
                       {t("player.stream")}
                     </span>
                   )}