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/ChannelPicker.tsx

@@ -71,7 +71,7 @@ export default function ChannelPicker({
           random to anyone who hasn't set a priority. Only while unfiltered: a search reorders nothing
           but does drop the rows that explain it. */}
       {!q && all.length > 0 && (
-        <div className="px-2 pt-1.5 text-[11px] text-muted">{t("sidebar.channelOrder")}</div>
+        <div className="px-2 pt-1.5 text-2xs text-muted">{t("sidebar.channelOrder")}</div>
       )}
       <div
         ref={fade.ref}