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/PlexInfo.tsx

@@ -360,7 +360,7 @@ export default function PlexInfo({
                     </div>
                     {c.role && (
                       <div
-                        className={`truncate text-[11px] ${overlay ? "text-white/55" : "text-muted"}`}
+                        className={`truncate text-2xs ${overlay ? "text-white/55" : "text-muted"}`}
                       >
                         {c.role}
                       </div>
@@ -420,7 +420,7 @@ export default function PlexInfo({
                         className="h-full w-full object-cover transition group-hover/col:scale-105"
                       />
                       {m.status === "watched" && (
-                        <span className="absolute right-1 top-1 rounded-sm bg-black/70 px-1 text-[9px] text-white">
+                        <span className="absolute right-1 top-1 rounded-sm bg-black/70 px-1 text-[0.5625rem] text-white">
                           ✓
                         </span>
                       )}
@@ -447,7 +447,7 @@ export default function PlexInfo({
                     >
                       {m.title}
                     </div>
-                    {m.year ? <div className="text-[11px] text-muted">{m.year}</div> : null}
+                    {m.year ? <div className="text-2xs text-muted">{m.year}</div> : null}
                   </button>
                 ))}
               </div>