web-player: single source of truth for theme, no light-mode flash

Remove the duplication of the ~18 per-hue dynamic tokens between
theme.css and buildThemeTokens(): theme.css now only defines fixed
neutrals, surfaces, layout and z-index; hue/mode-dependent values
come solely from tokens.ts, injected via SSR as an inline <style> in
+layout.svelte (tokensToCss helper) and reapplied on every hue
change. app.html sets data-theme before first paint by reading
localStorage, removing the dark flash when light mode was saved.

The two inline-style backgrounds become @utility
app-backdrop/app-vignette; the main's bottom padding becomes
@utility app-main, driven by --mf-dock-h via [data-has-track].

Note: <style>{expr}</style> doesn't interpolate in Svelte (it
compiles as component-scoped CSS) — the SSR block is injected with
{@html `<style>${css}</style>`} since the content is fully generated
(no user input).

Verified in the browser: --mf-accent/--mf-bg resolve correctly during
SSR, data-theme='light' applies before the first render with no
flash.

web-player/src/app.html

@@ -3,6 +3,13 @@
 	<head>
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<script>
+			try {
+				if (localStorage.getItem('musify.theme') === 'light') {
+					document.documentElement.dataset.theme = 'light';
+				}
+			} catch {}
+		</script>
 		%sveltekit.head%
 	</head>
 	<body data-sveltekit-preload-data="hover">