fix(a11y): express the type scale in rem so text resize does something

--yj-text-xs..xl were hardcoded px and are consumed by essentially
every component, so raising the OS or browser font size changed nothing
anywhere (WCAG 1.4.4, a11y.19). The values are identical at the default
16px root, and all six ui-visual baselines pass unchanged.

Verified in the running app rather than assumed: at a 24px root a track
cell goes 12px to 18px and a nav item 16px to 24px.

The same check confirms a11y.20, which is left unfixed and now
documented where the coupling lives: the row stays 33px while its text
grows to 18px, because four virtualized lists duplicate their row height
as the layout's _itemSize hint and carry contain: strict, which clips
rather than reflows. Fixing that means deriving _itemSize from a
measured row — a change to the scroll maths of four lists, not to a type
scale.

frontend/src/styles/tokens.css.ts

@@ -14,11 +14,29 @@ export const designTokens = css`
         --yj-icon-md: 18px;
         --yj-icon-lg: 24px;
 
-        /* ── Type scale ── */
-        --yj-text-xs: 11px;
-        --yj-text-sm: 12px;
-        --yj-text-md: 13px;
-        --yj-text-lg: 15px;
-        --yj-text-xl: 18px;
+        /* ── Type scale ──
+
+           In rem, so it tracks the root font size: the scale was
+           hardcoded px and consumed by essentially every component, so
+           raising the OS or browser font size changed nothing anywhere
+           (WCAG 1.4.4, a11y.19). The values are the same at the default
+           16px root — 11/16, 12/16, 13/16, 15/16, 18/16 — so nothing
+           moves until someone asks it to.
+
+           The scale is coupled to four virtualized lists and does not
+           reach them (a11y.20). track-list's rows are 33px, queue-panel's
+           49px and both playlist detail views' 45px, each duplicated as
+           the layout's _itemSize hint so the scroll maths agrees with
+           the DOM; all of them also carry contain: strict, which clips
+           overflow rather than growing the row. So larger text reflows
+           the app but crops those rows, and fixing that means deriving
+           _itemSize from a measured row rather than from a constant.
+           Deliberately not done here: it is a change to the scroll maths
+           of four lists, not a change to a type scale. */
+        --yj-text-xs: 0.6875rem;
+        --yj-text-sm: 0.75rem;
+        --yj-text-md: 0.8125rem;
+        --yj-text-lg: 0.9375rem;
+        --yj-text-xl: 1.125rem;
     }
 `;