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/AdminUsers.tsx

@@ -61,9 +61,7 @@ function Badge({
         ? "border-warn/40 text-warn"
         : "border-border text-muted";
   return (
-    <span className={`shrink-0 text-[11px] px-2 py-0.5 rounded-full border ${cls}`}>
-      {children}
-    </span>
+    <span className={`shrink-0 text-2xs px-2 py-0.5 rounded-full border ${cls}`}>{children}</span>
   );
 }
 
@@ -180,7 +178,7 @@ function UsersRoles({ me }: { me: Me }) {
                     {u.display_name ?? u.email.split("@")[0]}
                     {self && <span className="text-muted"> · {t("users.roles.you")}</span>}
                   </div>
-                  <div className="text-[11px] text-muted truncate">{u.email}</div>
+                  <div className="text-2xs text-muted truncate">{u.email}</div>
                 </div>
                 {u.role === "admin" && <Badge tone="accent">{t("users.roles.admin")}</Badge>}
                 {u.is_demo && <Badge tone="muted">{t("users.roles.demo")}</Badge>}
@@ -436,7 +434,7 @@ function AdminDemo() {
               <div className="min-w-0 flex-1">
                 <div className="text-sm truncate">{r.email}</div>
                 {r.added_by && (
-                  <div className="text-[11px] text-muted truncate">
+                  <div className="text-2xs text-muted truncate">
                     {t("settings.demo.addedBy", { who: r.added_by })}
                   </div>
                 )}
@@ -521,7 +519,7 @@ function InviteRow({
       <div className="min-w-0 flex-1">
         <div className="text-sm truncate">{inv.email}</div>
         {inv.requested_at && (
-          <div className="text-[11px] text-muted">
+          <div className="text-2xs text-muted">
             {t("settings.invites.requested", {
               time: new Date(inv.requested_at).toLocaleString(),
             })}