Dashboard SSE performance and reliability improvements

[unohee]

Summary

Improve the web dashboard (port 3847) initial load performance and SSE connection reliability.

Current Issues

• Initial page load can be slow due to inline HTML generation
• SSE connections may drop without automatic reconnection
• No compression for dashboard assets

Proposed Improvements

• Cache compiled dashboard HTML
• Add SSE heartbeat and auto-reconnect
• Gzip response compression
• Lazy-load non-critical dashboard sections

Status

Partially addressed in PR #22 (feat(dashboard): optimize initial load performance).