feat(app): render Mermaid code blocks - #43024

Summary

• Render fenced mermaid code blocks with Mermaid's own parser and SVG renderer, loaded only when a Mermaid fence appears.
• Keep the existing code block visible while streamed Mermaid is invalid; replace it after Mermaid successfully renders and restore code if the completed source is invalid.
• Preserve fenced-language metadata in completed Shiki output so Mermaid works both during streaming and after reloading an existing message.
• Import Mermaid through its public typed entrypoint; prebundle the linked Mermaid and KaTeX paths so Vite works with Bun's isolated linker.
• Upgrade UI to KaTeX 0.16.47 so UI and Mermaid share one resolved instance and one optimizer path.
• Match the active light/dark scheme and use Mermaid's strict security mode.

Validation

• bun typecheck in packages/session-ui
• bun test src/components/markdown-stream.test.ts — 23 passed
• bun --bun run build in packages/app
• Replayed the supplied exported message in 36-character chunks — both diagrams rendered
• Loaded the supplied exported message as an already completed response — both diagrams rendered

Requested by: @Hona (Luke Parker via Slack)