feat: add gstack dashboard — full UX visualizer for sprint pipeline, reports, and analytics

Self-contained Bun HTTP server + SPA dashboard that reads all gstack data sources
and provides visual insight into the sprint workflow. Dark/light theme toggle with
localStorage persistence. SVG-based charts, no external dependencies.

8 views: Overview, Pipeline, QA Reports, Performance, Canary Monitor, Design Review,
Analytics, Browser Sessions, Eureka Log. 14 API endpoints serving real-time data from
.gstack/ reports, ~/.gstack/analytics/, and browse daemon state.

Launch with: bun run dashboard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: HMAKT99