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)

tests/utils/omoConfig.test.ts

@@ -1,19 +1,9 @@
 import { describe, expect, it } from "vitest";
 import {
-  mergeOmoConfigPreview,
+  buildOmoProfilePreview,
   parseOmoOtherFieldsObject,
-  type OmoGlobalConfig,
 } from "@/types/omo";
 
-const EMPTY_GLOBAL: OmoGlobalConfig = {
-  id: "global",
-  disabledAgents: [],
-  disabledMcps: [],
-  disabledHooks: [],
-  disabledSkills: [],
-  updatedAt: "2026-01-01T00:00:00.000Z",
-};
-
 describe("parseOmoOtherFieldsObject", () => {
   it("解析对象 JSON", () => {
     expect(parseOmoOtherFieldsObject('{ "foo": 1 }')).toEqual({ foo: 1 });
@@ -29,22 +19,12 @@ describe("parseOmoOtherFieldsObject", () => {
   });
 });
 
-describe("mergeOmoConfigPreview", () => {
+describe("buildOmoProfilePreview", () => {
   it("只合并 otherFields 的对象值，忽略数组", () => {
-    const mergedFromArray = mergeOmoConfigPreview(
-      EMPTY_GLOBAL,
-      {},
-      {},
-      '["a", "b"]',
-    );
-    expect(mergedFromArray).toEqual({});
+    const fromArray = buildOmoProfilePreview({}, {}, '["a", "b"]');
+    expect(fromArray).toEqual({});
 
-    const mergedFromObject = mergeOmoConfigPreview(
-      EMPTY_GLOBAL,
-      {},
-      {},
-      '{ "foo": "bar" }',
-    );
-    expect(mergedFromObject).toEqual({ foo: "bar" });
+    const fromObject = buildOmoProfilePreview({}, {}, '{ "foo": "bar" }');
+    expect(fromObject).toEqual({ foo: "bar" });
   });
 });