feat(web): keep download actions discoverable

[devdudumuniz]

Summary

• keep add, remove, clear, and download actions visible while results are shown
• disable each action until its required selection or queue state is available
• keep queue actions visible when the queue is empty instead of returning early
• centralize action availability rules in a small tested helper

Compatibility

• no public API signature is removed or changed
• existing queue add, remove, clear, and download behavior is preserved

Local validation

• targeted web tests: 11 passed, 0 failed
• full Windows suite (Python 3.13): 1734 passed, 6 skipped, 18 warnings, 0 failed
• Docker Linux suite (Python 3.12): 1738 passed, 2 skipped, 16 warnings, 0 failed
• full pre-commit: passed (black, isort, flake8, mypy, pyupgrade, and repository hooks)
• Sphinx HTML with warnings as errors: passed
• Sphinx linkcheck: external HTTP 500 from https://dadosabertos.saude.gov.br/
• wheel and sdist build: passed; twine check passed with existing metadata warnings
• clean-environment wheel import smoke: passed
• Gitleaks PR-delta scan: no leaks found

Official CI reached the rebased head. Release checks passed; the main workflow was blocked by external HTTP 500/503 responses unrelated to this diff.

[codecov-commenter]

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Additional details and impacted files

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  Files           ?      179           
  Lines           ?    21810           
  Branches        ?        0           
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  Hits            ?    20790           
  Misses          ?     1020           
  Partials        ?        0           

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