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/Channels.tsx

@@ -748,7 +748,7 @@ export default function Channels() {
                   </span>
                 ))}
               </div>
-              <p className="text-[11px] text-muted mt-2">{t("channels.blocked.hint")}</p>
+              <p className="text-2xs text-muted mt-2">{t("channels.blocked.hint")}</p>
             </>
           )}
         </div>
@@ -814,7 +814,7 @@ function SyncBadge({ ok, label, hint }: { ok: boolean; label: string; hint?: str
   return (
     <Tooltip hint={hint ?? ""}>
       <span
-        className={`inline-flex items-center gap-1 text-[10px] px-1.5 py-0.5 rounded-full border ${
+        className={`inline-flex items-center gap-1 text-3xs px-1.5 py-0.5 rounded-full border ${
           ok ? "border-accent/40 text-accent" : "border-border text-muted"
         }`}
       >