[Shai34]

What & why

Buzz currently supports a single custom Whisper.cpp model, stored at a fixed
ggml-model-whisper-custom.bin path. Users who work with several fine-tuned
Whisper.cpp models can only keep one at a time.

A real example: transcribing Hebrew lectures, a fine-tuned Large-v3 Turbo
model is fast enough for everyday use, while a fine-tuned Large-v3 model is kept
for the occasional recording that needs higher accuracy. Both are ggml models;
today Buzz can only hold one as the custom model.

This PR generalises the single custom model into a small registry of custom
Whisper.cpp models, without touching the Whisper.cpp engine itself.

Changes

• Registry (buzz/settings/whisper_cpp_custom_models.py): any number of
  custom models {id, name, path, source_url}, persisted in the existing
  QSettings store.
• TranscriptionModel.custom_model_id: resolves to a registered model's
  path. When None, the legacy fixed path is used.
• Preferences UI: lists each custom model by name; add from a local file
  (registered in place, not copied) or a download URL; delete.
• Model selector: custom models are listed by name and selectable
  independently.
• Validation: a GGML/GGUF magic-header check rejects files Whisper.cpp
  can't load (technical only — it does not judge language/quality).
• CLI: --custom-model-id.
• Tests and docs.

Backward compatibility

• An existing ggml-model-whisper-custom.bin is surfaced automatically as
  "Custom Whisper.cpp Model" — not moved or re-downloaded.
• TranscriptionModel objects persisted by older versions (without
  custom_model_id) keep resolving to the legacy path.
• The Whisper.cpp engine and the language-detection plugin are unchanged;
  custom models stay excluded from automatic "largest model" selection.

Testing

Added unit tests for validation, the registry (add/get/remove, many models,
persistence), legacy compatibility, and custom-id path resolution.

[raivisdejus]

@Shai34 Thanks for your contribution

[codecov]

Codecov Report

❌ Patch coverage is 67.68061% with 85 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.57%. Comparing base (710870d) to head (4bcce68).

Files with missing lines	Patch %	Lines
...ts/preferences_dialog/models_preferences_widget.py	46.72%	65 Missing ⚠️
...ets/transcriber/transcription_options_group_box.py	70.73%	12 Missing ⚠️
buzz/settings/whisper_cpp_custom_models.py	91.35%	7 Missing ⚠️
buzz/model_loader.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1614      +/-   ##
==========================================
- Coverage   83.92%   83.57%   -0.35%     
==========================================
  Files         112      113       +1     
  Lines       12773    12990     +217     
==========================================
+ Hits        10720    10857     +137     
- Misses       2053     2133      +80     

Flag	Coverage Δ
Linux	82.80% <67.68%> (-0.32%)	⬇️
macOS	83.07% <67.68%> (-0.35%)	⬇️

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