fix: strengthen model fallback heading

[iamjr15]

Summary

• capitalizes the fallback heading as “Model Fallback”
• increases the heading to 12px with medium weight for clearer hierarchy
• preserves the shared double-shell status surface and all supporting copy sizing

Architecture

This remains presentation-only inside the existing ModelFallbackBlock. No status schema, model routing, or deployment boundary changes.

Decision

The heading receives the emphasis rather than increasing every line, keeping the operational details compact while making the card purpose immediately scannable.

Verification

• pnpm lint
• pnpm typecheck
• pnpm turbo build --force
• pnpm deadcode
• pnpm architecture:check
• pnpm turbo skills:build
• git diff --check
• production browser QA after merge