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/index.css

@@ -43,6 +43,15 @@
   --z-index-tuner: 9999;
 }
 
+/* Extra-small text steps in rem, so these labels scale with --font-scale like every other
+   text (the ~124 px-frozen text-[10/11px] labels did NOT). Font-size ONLY — no paired
+   --text-*--line-height — to match the `text-[Npx]` arbitraries they replace and avoid any
+   vertical-rhythm shift. */
+@theme {
+  --text-2xs: 0.6875rem; /* 11px @ base — the most common frozen label */
+  --text-3xs: 0.625rem; /* 10px @ base */
+}
+
 /*
   The default border color has changed to `currentcolor` in Tailwind CSS v4,
   so we've added these compatibility styles to make sure everything still
@@ -79,10 +88,11 @@
 :root {
   --font-scale: 1.06;
 
-  /* Vertical space the floating top header needs to clear (top offset 12px + pill height 44px +
-     bottom breathing room ~12px). Module content pads its top by this so nothing hides under the
-     header; kept in one place since App and Playlists both reference it. */
-  --hdr-h: 68px;
+  /* Vertical space the floating top header needs to clear (top offset ~0.75rem + pill height ~2.75rem
+     + bottom breathing room ~0.75rem). In rem so it grows with --font-scale in lock-step with the
+     rem-sized header pills — a px value let content slide under the header at scale 1.3. Kept in one
+     place since App and Playlists both reference it. */
+  --hdr-h: 4.25rem;
 
   /* ===== Glass tunables — every value that materially drives the look lives here so it can be
      tuned in ONE place. ===== */