perf(ui): deduplicate refreshes and poll active jobs only [4/5]

[Anthony Cintron (prbatero)]

Summary

Part 4 of 5 in the project layer-loading performance stack. Stop duplicate and overlapping project-detail requests and make background refresh conditional on actual active work.

Changes

• Add response-aware JSON GET handling with ETag and bodyless 304 support.
• Share same-project in-flight requests and abort superseded project requests.
• Force refresh after mutations while allowing normal navigation to use a fresh cache.
• Skip state updates for 304 responses.
• Poll only while the tab is visible, no request is running, and at least one job is active.

Testing

• Full UI suite: 114 passed.
• Every changed UI file passes ESLint with zero warnings.
• Production Vite build passed.

Stack

• Depends on: perf(project-details): optimize loading and conditional caching [3/5] #187.
• This PR: 4/5, based on prbatero/feat/perf-project-details-api.
• Next branch: prbatero/feat/perf-ui-route-loading.

Review Notes

This PR intentionally does not change route loading or Azure Maps asset delivery; those changes are isolated in PR 5.