fix: allow queuing transfers during active copies

[bifrost0x]

Summary

• keep cross-source copy actions available while another server copy is active
• enqueue selected items immediately so pending work is visible and starts sequentially
• serialize server copies with upload/download queue owners and preserve source lifetimes
• limit conflict-dialog cancellation to the affected batch while preserving later batches

Fixes #184

Validation

• node --test tests/js/*.test.js (373 passed)
• npm run lint:js
• npm run vendor:check
• Firefox regression scenario for the File Manager transfer queue (1 passed)
• File Workspace E2E suite in Chromium (8 passed)

[chatgpt-codex-connector]

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T19:01:36.813905Z	d1449c9	PR opened

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[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: d1449c9f87

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