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/Messages.tsx

@@ -165,7 +165,7 @@ function ConversationList({
                       >
                         {c.partner.name}
                       </span>
-                      <span className="ml-auto text-[11px] text-muted shrink-0">
+                      <span className="ml-auto text-2xs text-muted shrink-0">
                         {relativeTime(c.last_message.created_at)}
                       </span>
                     </div>
@@ -176,7 +176,7 @@ function ConversationList({
                         {previewText(c)}
                       </span>
                       {c.unread > 0 && (
-                        <span className="ml-auto shrink-0 min-w-5 h-5 px-1.5 grid place-items-center rounded-full bg-accent text-accent-fg text-[11px] font-semibold">
+                        <span className="ml-auto shrink-0 min-w-5 h-5 px-1.5 grid place-items-center rounded-full bg-accent text-accent-fg text-2xs font-semibold">
                           {c.unread}
                         </span>
                       )}