feat(voxtype): select an installed local model

[quanru]

Summary

• Show the active Voxtype engine and model and list installed models in Doubao Say settings.
• Selecting another installed model updates Voxtype's own configuration and restarts its daemon, including for SenseVoice where record --model is ignored.
• Keep the default choice bound to Voxtype's active model. Document that a model switch also affects Voxtype's own hotkey.

Dependency

Stacked on #49 (feat/voxtype-controls); review and merge #48, then #49, then this PR.

Verification

• make check PYTHON=/home/leyang/doubao-say/.venv/bin/python passed.
• make coverage PYTHON=/home/leyang/doubao-say/.venv/bin/python passed (61% total).
• Midscene on an isolated Xvfb desktop opened Settings and confirmed sensevoice, sensevoice-small, and the model selector. Local evidence: artifacts/verification/voxtype-model-choice/after/model-selector.png and verification.md (ignored by Git).
• Read-only check against the installed Voxtype 1.0.1 daemon confirmed sensevoice-small is active and installed; selecting that already active model returned without restarting.

The machine currently has one installed SenseVoice model. The test suite simulates switching to a second installed model; a real second-model switch has not been exercised.