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/database/dao/mod.rs

@@ -4,7 +4,6 @@
 
 pub mod failover;
 pub mod mcp;
-pub mod omo;
 pub mod prompts;
 pub mod providers;
 pub mod proxy;
@@ -16,4 +15,3 @@ pub mod universal_providers;
 // 所有 DAO 方法都通过 Database impl 提供，无需单独导出
 // 导出 FailoverQueueItem 供外部使用
 pub use failover::FailoverQueueItem;
-pub use omo::OmoGlobalConfig;