fix(web): enlarge model fallback heading

[iamjr15]

Why

The fallback label was capitalized correctly but remained 12px in production, so it did not provide the requested heading hierarchy over the fallback details.

What changed

• renders Model Fallback at 14px
• uses the primary thread text token so the heading has clear visual hierarchy
• preserves the unified double-shell status treatment

Verification

• pnpm exec biome check apps/web/src/components/chat/message-parts.tsx
• pnpm --filter @cheatcode/web typecheck
• pnpm --filter @cheatcode/web build
• production computed-style inspection established the previous heading was 12px