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/Sidebar.tsx

@@ -54,7 +54,7 @@ function TagChip({
     >
       <span>{tag.name}</span>
       <span
-        className={`tabular-nums text-[10px] leading-none px-1 py-0.5 rounded-full ${
+        className={`tabular-nums text-3xs leading-none px-1 py-0.5 rounded-full ${
           active ? "bg-accent-fg/20 text-accent-fg" : "bg-muted/15 text-muted"
         }`}
       >
@@ -232,7 +232,7 @@ export default function Sidebar({
                     onClick={() =>
                       setFilters({ ...filters, dateFrom: undefined, dateTo: undefined })
                     }
-                    className="text-[11px] text-muted hover:text-accent self-end"
+                    className="text-2xs text-muted hover:text-accent self-end"
                   >
                     {t("sidebar.clearDates")}
                   </button>
@@ -255,7 +255,7 @@ export default function Sidebar({
               />
             ))}
             {facetsReady && chips.length === 0 && (
-              <span className="text-[11px] text-muted">{t("sidebar.noMatchingTags")}</span>
+              <span className="text-2xs text-muted">{t("sidebar.noMatchingTags")}</span>
             )}
           </div>
         );
@@ -268,11 +268,11 @@ export default function Sidebar({
               className="flex items-center justify-between mb-1.5"
               title={t("sidebar.tagModeTooltip")}
             >
-              <span className="text-[10px] uppercase tracking-wide text-muted">
+              <span className="text-3xs uppercase tracking-wide text-muted">
                 {t("sidebar.match")}
               </span>
               <div
-                className="flex items-center rounded-full border border-border bg-card p-0.5 text-[10px]"
+                className="flex items-center rounded-full border border-border bg-card p-0.5 text-3xs"
                 role="group"
                 aria-label={t("sidebar.match")}
               >
@@ -303,7 +303,7 @@ export default function Sidebar({
                 />
               ))}
               {facetsReady && chips.length === 0 && (
-                <span className="text-[11px] text-muted">{t("sidebar.noMatchingTags")}</span>
+                <span className="text-2xs text-muted">{t("sidebar.noMatchingTags")}</span>
               )}
             </div>
           </>
@@ -323,7 +323,7 @@ export default function Sidebar({
               />
             ))}
             {facetsReady && tagChips.length === 0 && (
-              <span className="text-[11px] text-muted">{t("sidebar.noMatchingTags")}</span>
+              <span className="text-2xs text-muted">{t("sidebar.noMatchingTags")}</span>
             )}
             <button
               onClick={() => setTagManagerOpen(true)}