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/docs/frontend-refactor-plan.md

@@ -774,7 +774,7 @@ Tareas:
 | ----- | ------ | ------ | -------------------------- |
 | P0    | ✅     | (pendiente) | Button primary/secondary se mantienen iguales (decisión del usuario) |
 | P1    | ✅     | (pendiente) | Verificado con `build` (Tailwind compila `@apply sm:` dentro de `@utility`); sin QA visual autenticada (Docker no disponible en esta sesión) |
-| P2    | ⬜     |        |                            |
+| P2    | ✅     | (pendiente) | `<style>{expr}</style>` no interpola en Svelte (se trata como CSS del componente); solución: `{@html \`<style>${css}</style>\`}` |
 | P3    | ⬜     |        |                            |
 | P4    | ⬜     |        |                            |
 | P5    | ⬜     |        |                            |