Bloque J2 revert — drop frontend runtime validators

Removes the `VALID_FRONTEND_DENSITIES`/`VALID_FRONTEND_MODES`/
`VALID_FRONTEND_DIRS` const triplet introduced by Bloque J2. The
matching `assertValidFrontendValue` helper and its three call sites
in `active-frontend.svelte.ts` were already cleaned up earlier in
the working tree.

Validation moves to `libs/prefs/validate-intent.ts` once the prefs
artifact lands — the design at `src/arts/prefs/README.md` puts every
`setIntent(...)` write through one shared validator instead of
spreading per-art runtime guards. J1 (lang `SvelteSet → Set`) and J3
(sium `CodeError` migration) stay; only the J2 portion of the
combined commit is reverted.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/arts/frontend/consts.ts

@@ -10,11 +10,3 @@ export const FRONTEND_ATTRS = {
 	REDUCED_SOUND: 'data-reduced-sound',
 	DENSITY: 'data-density'
 } as const;
-
-// Closed-set values for setters that take a string union. Used by the
-// active wrapper to assert in DEV and silently coerce in PROD when JS
-// callers pass an out-of-spec value (TypeScript users get a compile
-// error first; this guards untyped imports / formless inputs).
-export const VALID_FRONTEND_DENSITIES = ['compact', 'normal', 'comfortable'] as const;
-export const VALID_FRONTEND_MODES = ['light', 'dark', 'auto'] as const;
-export const VALID_FRONTEND_DIRS = ['ltr', 'rtl', 'auto'] as const;