fix(web): open archived sessions from sidebar

[he-yufeng]

Summary

• let archived sessions resolve as the current session when selected
• keep the URL/session validator from clearing a selected archived session
• wait for archived-session pagination/loading before deciding a selected session is missing

Fixes #2312

To verify

• npm ci
• npm run typecheck
• npm run lint
• npm run build
• git diff --check

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Do not block URL validation on archived pagination state

Guarding validation with hasMoreArchivedSessions causes ?session=... cleanup to be skipped indefinitely whenever the user has at least one additional archived page (e.g. >100 archived sessions) but never clicks “Load more”. In that case an invalid or deleted session ID remains selected forever, because this effect never reaches the sessionExists check even after initial archived loading finishes; previously this path only depended on active-session pagination.

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