feat(brand): DST vector mascot with black/brand canvas modes

[implemented.stream (thanabartbb)]

Adds DstMascot (canvas="black"|"brand"|"transparent"), DstMascotThemed, dstMascotCanvasFromColorScheme, demo /design/dst-mascot.

• Branch already contained current main (no conflicts).
• Fixed lint error (setState in effect) by switching DstMascotThemed to useSyncExternalStore.
• Animations gated by prefers-reduced-motion: no-preference; label → role=img + aria-label.
• Reference recorded in docs/design/ui-ux-revamp-plan.md.
• Checks: lint, typecheck, 347 tests, next build (Node 20); OpenNext build + local workerd preview: /, /design/dst-mascot, /plugins → 200.
• Note: after PR feat: agent readiness: Markdown negotiation, OpenAPI, OAuth metadata, JSON API 404s, MCP discovery #26 merges, add /design/dst-mascot to STATIC in src/lib/agent/site-paths.ts.

Not merged, not deployed.

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