fix(admin): coalesce session list refresh and fix initial loading

[gelbh]

Summary

• Set loading true on the same render when enabled flips true so /admin no longer flashes “No live sessions” before the first fetch.
• Coalesce overlapping full refreshes onto one in-flight promise, with a trailing refresh when another caller arrives mid-flight (StrictMode / Refresh / join).
• Add hook tests for enabled flip, re-enable, trailing refresh, background retry after error, and empty success.

Test plan

• npx vitest related --run --passWithNoTests src/hooks/admin/useAdminSessionList.ts
• Open /admin signed in — skeleton until first settle; no empty flash
• Rapid Refresh / StrictMode — list still populates

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 484aad6 and 6cbd834.

📒 Files selected for processing (2)

• src/hooks/admin/useAdminSessionList.test.ts
• src/hooks/admin/useAdminSessionList.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/admin-list-initial-load

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