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/DownloadCenter.tsx

@@ -227,7 +227,7 @@ function DownloadRow({
       <div className="min-w-0 flex-1">
         <div className="font-medium leading-snug line-clamp-1 flex items-center gap-1.5">
           {job.job_kind === "edit" && (
-            <span className="inline-flex items-center gap-1 shrink-0 px-1.5 py-0.5 rounded-sm text-[10px] font-semibold uppercase tracking-wide bg-accent/15 text-accent">
+            <span className="inline-flex items-center gap-1 shrink-0 px-1.5 py-0.5 rounded-sm text-3xs font-semibold uppercase tracking-wide bg-accent/15 text-accent">
               <Scissors className="w-3 h-3" /> {t("downloads.clipBadge")}
             </span>
           )}
@@ -280,7 +280,7 @@ function DownloadRow({
                     style={{ width: `${job.progress}%` }}
                   />
                 </div>
-                <div className="text-[11px] text-muted mt-0.5 flex gap-2">
+                <div className="text-2xs text-muted mt-0.5 flex gap-2">
                   <span>{job.progress}%</span>
                   {job.speed_bps ? <span>{formatSpeed(job.speed_bps)}</span> : null}
                   {job.eta_s ? <span>{formatEta(job.eta_s)}</span> : null}