Remove built-in ThemeSeeds in favor of consumer components

[abernier]

Summary

Remove the built-in ThemeSeeds selector and seed table now owned by the design-system documentation. This is the counterpart to pmndrs/design-system#73, using the consumer components API from #713.

 MDX rendering
-  built-in ThemeSeeds + fragment fallback
+  consumer-registered components

 authoring/theme-seeds
-  embedded selector documentation
+  redirect to design-system/tokens/colors#seeds

Keep the shared theme hooks, footer controls and stored preset support used by the local component. Rewrite the persistence browser tests against remaining documentation pages. A patch changeset records the removal and consumer configuration migration; design-system was the only consumer.

Evidence

• Before: the renderer owned the selector, table, fragment fallback and dedicated authoring page.
  After: all four are removed; design-system owns its interactive and static renderers.
• pnpm run lgtm: 656 tests passed, 4 skipped; types, lint, formatting and registry validation passed.
• Pre-merge pnpm run LGTM: passed on 94a141f; 656 unit tests passed (4 skipped), all 118 Playwright tests passed, including Purple and Lime persistence at mobile and desktop widths. Four-axis review found no issues requiring changes.
• Built the CLI and rendered all 11 design-system pages with its local static component against this checkout after removing the built-in renderer.
• Local validation history: an overlapping static build temporarily moved API/live sources and caused a typecheck failure; validation was rerun after restoration. The first browser run exposed a test helper hydration race on desktop; the helper now distinguishes the mobile breakpoint and awaits the controls.

Merge Danger

Door: two-way

The change is reversible and ships as a patch: design-system was the only consumer and is migrated in the linked PR.

Blast Radius: consumers

MDX using <ThemeSeeds /> must register its own component. The design-system counterpart now provides both renderers and stays pinned to docs 4.25.0 until a separate upgrade. Existing theme preferences and generic consumer components remain supported. The authoring URL redirect applies to Next server deployments; static exports do not apply Next redirects.