web: refine navigation and transcript surface

[PerishCode]

Summary

• Refine shell density, panel geometry, navigation, and Inspect interactions.
• Add event-first navigation and Inspect panel intents.
• Rework transcript rendering into speaker groups with anchored avatars, divider timing, and hover outlines.
• Add controlled markdown rendering for regular transcript messages.

Validation

• pnpm lint
• pnpm build
• commit hook: cargo fmt, flavor, sidecar doctor, cargo clippy, cargo test, contracts codegen, lint, web build
• browser screenshots captured during transcript and markdown validation