* fix(settings): apply already-downloaded models off the GTK main loop
Changing Model Size, Specialization, or Language froze Settings when the
model was already on disk, because reconfigure ran in the changed handler.
Move that apply onto a worker thread (same shape as the download path) and
keep the apply-guard held until GLib.idle_add clears it so a second pick
cannot start a parallel load.
Fixes#789.
* fix(settings): guard Test/apply while downloaded-model worker runs
Block apply_settings and Test Dictation while _applying_settings is held
so a concurrent Test click cannot race the shared engine. Annotate the
new worker/helpers per AGENTS.md and cover the no-op/block behavior.
* fix(settings): resync pickers when a second pick lands mid-apply
While an already-downloaded model worker was running, Model Size /
Specialization / Language could still move. The apply guard blocked
saving the new pick, then finish only cleared the flag, so the UI
could show B with dictation still on A. On finish, if the selection
no longer matches the saved config, put the pickers back. Idle
resyncs from workers now skip a destroyed dialog the same way update
check does.
* fix(settings): always resync pickers after model apply finishes
A second Model Size pick on whisper.cpp could move the size combo
while the apply guard skipped rebuilding specialization, so selected
settings still looked like the saved variant and finish skipped the
resync. Always put the pickers back on the saved config when the
worker ends, and restore language from that config too.
* test(settings): prove mid-apply picker handlers round-trip on finish
Call the real size, specialization, and language handlers while the
apply guard is held, and run finish through the real resync path so
whisper.cpp size and language cannot silently stick on the second pick.
