Refactor dashboard server modules

[Octane0411]

Problem statement

src/observability/dashboard-server.ts had grown into a single large file responsible for routing, SSE control, HTML generation, CSS, client-side rendering, and formatting helpers. That made the dashboard harder to evolve and reason about safely.

Scope summary

• split dashboard server responsibilities into focused observability modules
• extract shared formatting helpers
• extract HTTP/snapshot helper utilities
• extract the SSE live updates controller
• extract dashboard rendering/template assembly
• keep dashboard behavior and external routes unchanged

References

• Dashboard entrypoint: /Users/wangruobing/Personal/symwork/symphony/src/observability/dashboard-server.ts
• Upstream dashboard view: /Users/wangruobing/Personal/symwork/original-symphony/elixir/lib/symphony_elixir_web/live/dashboard_live.ex
• Upstream dashboard stylesheet: /Users/wangruobing/Personal/symwork/original-symphony/elixir/priv/static/dashboard.css

Validation

• pnpm test
• pnpm typecheck
• pnpm lint