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

Validation

• pytest -q pysus/tests/web/test_download_queue.py (3 passed)
• black --check pysus/download_queue.py pysus/web/pages/1_client.py pysus/tests/web/test_download_queue.py
• flake8 pysus/download_queue.py pysus/web/pages/1_client.py pysus/tests/web/test_download_queue.py
• python -m compileall -q pysus/web pysus/download_queue.py
• official CI: 13 checks passed

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Additional details and impacted files

@@           Coverage Diff           @@
##             main     #338   +/-   ##
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  Coverage        ?   95.24%           
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  Files           ?      171           
  Lines           ?    20053           
  Branches        ?        0           
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  Hits            ?    19100           
  Misses          ?      953           
  Partials        ?        0           

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