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/components/providers/forms/helpers/opencodeFormUtils.ts

@@ -1,5 +1,4 @@
 import type { OpenCodeModel, OpenCodeProviderConfig } from "@/types";
-import type { OmoGlobalConfig } from "@/types/omo";
 import type { PricingModelSourceOption } from "../ProviderAdvancedConfig";
 
 // ── Default configs ──────────────────────────────────────────────────
@@ -52,15 +51,6 @@ export const OPENCLAW_DEFAULT_CONFIG = JSON.stringify(
   2,
 );
 
-export const EMPTY_OMO_GLOBAL_CONFIG: OmoGlobalConfig = {
-  id: "global",
-  disabledAgents: [],
-  disabledMcps: [],
-  disabledHooks: [],
-  disabledSkills: [],
-  updatedAt: "",
-};
-
 // ── Pure functions ───────────────────────────────────────────────────
 
 export function isKnownOpencodeOptionKey(key: string): boolean {