* fix(startup): resolve model size per engine instead of the legacy key
Startup read the generic "model_size" key, which always holds whichever
engine was saved last, so it initialized the configured engine with
another engine's model. With engine=whisper_cpp and a config left at
model_size=medium by a VOSK save, startup looked for ggml-medium.bin,
reported the model as missing and offered a fresh download while the
working ggml-small.bin named by whisper_cpp_model_size sat on disk.
Use ConfigManager.get_model_size_for_engine(), the resolver the settings
dialog already uses, so the generic key stays a write-only compatibility
shim. It falls back to the generic key when no per-engine key exists, so
older configs keep working.
* fix(config): migrate model_size into whisper_cpp_model_size
Review follow-up on #684.
get_model_size_for_engine() only falls back to the generic model_size key
when the engine key is missing from the in-memory dict, and DEFAULT_CONFIG
already ships whisper_cpp_model_size. After the merge in load_config() the
key is therefore always present, so a user file holding only
{engine: whisper_cpp, model_size: small} used to start on small and would
now start on the default tiny — the older-config path from #681.
_migrate_config already copies the model into vosk_model_size and
whisper_model_size; do the same for whisper_cpp so the resolver change is
behaviour-preserving for those files, and widen the migration trigger to
notice a missing whisper_cpp key.
Tests run against a real ConfigManager on a temp config, not a mocked
resolver: the migration case fails without this change, and the two
resolver cases pin the engine key winning over the generic one.
