fix: retry transient HTTP statuses and show friendly download errors

[bigcat88]

Summary

Fixes #430.

Three focused changes that together address the bug report (visual glitches + verbose tracebacks when pasting many comfy model download commands into a shell):

• Retry transient HTTP statuses. _download_file_httpx now raises a private _TransientHTTPStatusError for statuses in {408, 429, 500, 502, 503, 504}, which the existing retry loop catches alongside network-level errors. CivitAI CDN blips and other transient server errors can now self-heal instead of aborting the download on the first attempt.
• Friendly error rendering. comfy model download catches DownloadException and prints a one-line red error before exiting with code 1, replacing the previous ~100-line Rich-panel Python traceback. comfy node install catches it the same way, using the existing ui.display_error_message path.
• Transient progress bars. ui.show_progress and _poll_aria2_download now use transient=True, so each progress bar is erased when its download completes. Running many comfy model download commands in sequence no longer leaves 30 persistent bars stacked in the terminal.

Notes / follow-ups

• Retry-After header on 429 responses is not honored; fixed 2 s / 4 s backoff is used. Worth a follow-up if we see CivitAI or similar providers sending it.
• HuggingFace downloads (huggingface_hub.hf_hub_download) are not touched — they have their own error semantics.
• A pre-existing raise DownloadException("Filename cannot be empty") at comfy_cli/command/models/models.py:315 still produces a traceback because it sits above the new try/except; unrelated to [Potential bug] Visual glitches and error #430 and can be cleaned up separately.

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
comfy_cli/command/custom_nodes/command.py	0.00%	6 Missing ⚠️

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
+ Coverage   76.63%   76.66%   +0.03%     
==========================================
  Files          35       35              
  Lines        4279     4299      +20     
==========================================
+ Hits         3279     3296      +17     
- Misses       1000     1003       +3     

Files with missing lines	Coverage Δ
comfy_cli/command/models/models.py	72.68% <100.00%> (+0.46%)	⬆️
comfy_cli/file_utils.py	89.56% <100.00%> (+1.17%)	⬆️
comfy_cli/ui.py	66.66% <100.00%> (ø)
comfy_cli/command/custom_nodes/command.py	80.84% <0.00%> (-0.79%)	⬇️

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