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/src/lib/theme/tokens.ts

@@ -67,9 +67,6 @@ export function buildThemeTokens(hue: number, mode: 'dark' | 'light' = 'dark'):
 		'--mf-accent-btn-bg-hover': `oklch(${r(accentL)} ${r(accentC)} ${H} / 0.24)`,
 		'--mf-bg': light ? `oklch(0.97 0.006 ${H})` : `oklch(0.068 0.008 ${H})`,
 		'--mf-elevated': light ? `oklch(0.99 0.003 ${H})` : `oklch(0.11 0.012 ${H})`,
-		'--mf-sidebar-bg': light
-			? `linear-gradient(180deg, oklch(0.985 0.004 ${H}), oklch(0.965 0.006 ${H}))`
-			: `linear-gradient(180deg, oklch(0.095 0.012 ${H}), oklch(0.068 0.007 ${H}))`,
 		'--mf-panel-bg': light ? `oklch(0.97 0.006 ${H} / 0.7)` : `oklch(0.09 0.01 ${H} / 0.28)`,
 		'--mf-bar-bg': light ? `oklch(0.97 0.006 ${H} / 0.9)` : `oklch(0.105 0.012 ${H} / 0.9)`,
 		'--mf-hairline': light ? `oklch(0.3 0.02 ${H} / 0.1)` : `oklch(0.62 0.03 ${H} / 0.08)`,