fix(deps): remove mermaid as hard runtime dependency

[sleitor]

Problem

Fixes #501

mermaid is listed in dependencies of the core streamdown package, but it is never actually imported at runtime — only import type { MermaidConfig } appears in source files. The actual mermaid runtime is exclusively owned by the optional @streamdown/mermaid plugin package.

This forces ~75 MB of mermaid into every install of streamdown, even for users who have no interest in diagram rendering.

Fix

1. Remove mermaid from dependencies in packages/streamdown/package.json.

2. Replace import type { MermaidConfig } from 'mermaid' in three files with a local structural type:

   // biome-ignore lint/suspicious/noExplicitAny: structural pass-through for mermaid config
   export type MermaidConfig = Record<string, any>;

   Since the config is only ever passed through to the plugin (never inspected in core), this is fully compatible. Users who want exhaustive type-safety can still import type { MermaidConfig } from 'mermaid' — TypeScript's structural typing ensures it remains assignable.

3. Add changeset (patch bump for streamdown).

Verification

• pnpm build ✅ — dist/index.d.ts no longer contains from 'mermaid'
• pnpm test ✅ — 982/982 tests pass
• pnpm biome check ✅

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Additional Suggestion:

Two source files still import MermaidConfig from the removed "mermaid" package, which will break dts generation and published type declarations.

[sleitor]

Good catch from VADE! Fixed in 56e02a1 — download-button.tsx, fullscreen-button.tsx, and mermaid.test.tsx now all import MermaidConfig from the local ../plugin-types (or ../lib/plugin-types for the test). No more coupling to the mermaid package in any of the source files.