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/Welcome.tsx

@@ -554,7 +554,7 @@ function AuthCard() {
               >
                 {t("welcome.auth.resendLink")}
               </button>
-              <div className="flex items-center gap-3 my-4 text-[11px] uppercase tracking-wide text-muted">
+              <div className="flex items-center gap-3 my-4 text-2xs uppercase tracking-wide text-muted">
                 <span className="h-px flex-1 bg-border" />
                 {t("welcome.auth.or")}
                 <span className="h-px flex-1 bg-border" />
@@ -576,7 +576,7 @@ function AuthCard() {
                 </button>
               ) : (
                 <form onSubmit={onDemo} className="mt-2 flex flex-col gap-2">
-                  <p className="text-[11px] text-muted">{t("welcome.auth.demoHint")}</p>
+                  <p className="text-2xs text-muted">{t("welcome.auth.demoHint")}</p>
                   <div className="flex gap-2">
                     <input
                       type="email"