refactor: remove OMO common config two-layer merge system

Each OMO provider now stores its complete configuration directly in
settings_config.otherFields instead of relying on a shared OmoGlobalConfig
merged at write time. This simplifies the data flow from a 4-tuple
(agents, categories, otherFields, useCommonConfig) to a 3-tuple and
eliminates an entire DB table, two Tauri commands, and ~1700 lines of
merge/sync code across frontend and backend.

Backend:
- Delete database/dao/omo.rs (OmoGlobalConfig struct + get/save methods)
- Remove get/set_config_snippet from settings DAO
- Remove get/set_common_config_snippet Tauri commands
- Replace merge_config() with build_config() in services/omo.rs
- Simplify OmoVariant (remove config_key, known_keys)
- Simplify import_from_local and build_local_file_data
- Rewrite all OMO service tests

Frontend:
- Delete OmoCommonConfigEditor.tsx and OmoGlobalConfigFields.tsx
- Delete src/lib/api/config.ts
- Remove OmoGlobalConfig type and merge preview functions
- Remove useGlobalConfig/useSaveGlobalConfig query hooks
- Simplify useOmoDraftState (remove all common config state)
- Replace OmoCommonConfigEditor with read-only JsonEditor preview
- Clean i18n keys (zh/en/ja)

src-tauri/src/services/provider/mod.rs

@@ -525,9 +525,8 @@ impl ProviderService {
                 .set_omo_provider_current(app_type.as_str(), id, "omo-slim")?;
             crate::services::OmoService::write_config_to_file(state, &crate::services::omo::SLIM)?;
             // OMO ↔ OMO Slim mutually exclusive: remove Standard config
-            let _ = crate::services::OmoService::delete_config_file(
-                &crate::services::omo::STANDARD,
-            );
+            let _ =
+                crate::services::OmoService::delete_config_file(&crate::services::omo::STANDARD);
             return Ok(SwitchResult::default());
         }