fix(stt): auto-detect .en and other ggml model variants

[ElbertePlinio]

Found by the #66 macOS hardware E2E: detect_model_path checked a fixed filename list without .en variants, so ggml-base.en.bin — the standard English model name whisper.cpp docs point users at — was never detected and dictation failed with "no whisper model found" despite the model being present. Affects Linux identically.

• Preference list extended with small.en/base.en/tiny.en
• New deterministic fallback: any ggml-*.bin in the model dir, sorted by name
• Env override and Arch /usr/share fallback untouched
• Detection logic extracted to a pure detect_model_in_dir helper (testable without process-global HOME mutation) with 4 new tests

Live-verified on this machine: detection now resolves ~/.local/share/whisper.cpp/models/ggml-base.en.bin.

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