Improve dashboard caching and preloading

[stylessh]

Summary

• scope GitHub cache dedupe per request and move auth/session resolution into shared server runtime helpers to avoid Workers cross-request issues and reduce per-request setup cost.
• speed dashboard navigation with route loaders, eager tab preloading, one-shot hover prefetch, and a fixed overflow-scrolling topbar tab strip.
• persist tab-scoped PR and issue page queries, aggregate detail payloads server-side, and keep dynamic data fresh with short TTLs plus client revalidation for PR status and activity data.