fix(config): use Hermes config.yaml as single source of default model (#773)

Removes split-brain where WebUI Settings persisted default_model separately from Hermes runtime config.yaml. New POST /api/default-model endpoint writes to config.yaml. Existing saved values migrated on first load.

Fixes #761

Co-authored-by: aronprins <aronprins@users.noreply.github.com>

tests/test_batch_fixes.py

@@ -219,7 +219,8 @@ class TestSystemTheme:
     def test_panels_hydrates_appearance_before_models_fetch(self):
         src = read("static/panels.js")
         skin_idx = src.index("const skinVal=(settings.skin||'default').toLowerCase();")
-        models_idx = src.index("const models=await api('/api/models');")
+        # models is now declared as let models=null before the try block
+        models_idx = src.index("models=await api('/api/models');")
         assert skin_idx < models_idx, (
             "loadSettingsPanel must hydrate theme/skin before awaiting /api/models, "
             "otherwise a slow model fetch can clobber an in-progress skin selection"