web-player: base design system tokens

Add, additively (no component migration yet), the tokens from
section B.1 of the refactor plan: radii (tag/thumb), cover and icon
sizes, container widths, typographic tracking/leading, button colors
(--color-btn*), typography @utility recipes (text-display-*,
text-eyebrow, text-body, text-count), icon stroke
(stroke-thin/regular/bold), surfaces/grids (glass-bar, grid-cards*,
theme-transition), and the .animate-enter stagger via --i. Add
layout variables (--mf-player-side-w, --mf-menu-w, --mf-submenu-w,
--mf-range-*) and semantic z-index in theme.css. Clean up unused
color utilities (--color-accent-dim/glow/hair/line/muted,
--color-on-accent) and remove --mf-sidebar-bg (no importers) from
theme.css and tokens.ts. Scrollbar moves to tokens with a light-mode
variant.

Verified with npm run build (Tailwind compiles the @utility recipes
with responsive variants inside @apply).

web-player/docs/frontend-refactor-plan.md

@@ -773,7 +773,7 @@ Tareas:
 | Parte | Estado | Commit | Notas / decisiones tomadas |
 | ----- | ------ | ------ | -------------------------- |
 | P0    | ✅     | (pendiente) | Button primary/secondary se mantienen iguales (decisión del usuario) |
-| P1    | ⬜     |        |                            |
+| P1    | ✅     | (pendiente) | Verificado con `build` (Tailwind compila `@apply sm:` dentro de `@utility`); sin QA visual autenticada (Docker no disponible en esta sesión) |
 | P2    | ⬜     |        |                            |
 | P3    | ⬜     |        |                            |
 | P4    | ⬜     |        |                            |