✨ feat(sync): multi-language sync wizard, backend locale support, and conflict modal UX improvements

Frontend (web)
- ModelsActions.jsx
  - Replace “Sync Official” with “Sync” and open a new two-step SyncWizard.
  - Pass selected locale through to preview, sync, and overwrite flows.
  - Keep conflict resolution flow; inject locale into overwrite submission.

- New: models/modals/SyncWizardModal.jsx
  - Two-step wizard: (1) method selection (config-sync disabled for now), (2) language selection (en/zh/ja).
  - Horizontal, centered Radio cards; returns { option, locale } via onConfirm.

- UpstreamConflictModal.jsx
  - Add search input (model fuzzy search) and native pagination.
  - Column header checkbox now only applies to rows in the current filtered result.
  - Fix “Cannot access ‘filteredDataSource’ before initialization”.
  - Refactor with useMemo/useCallback; extract helpers to remove duplicated logic:
    - getPresentRowsForField, getHeaderState, applyHeaderChange
  - Minor code cleanups and stability improvements.

- i18n (en.json)
  - Add strings for the sync wizard and related actions (Sync, Sync Wizard, Select method/source/language, etc.).
  - Adjust minor translations.

Hooks
- useModelsData.jsx
  - Extend previewUpstreamDiff, syncUpstream, applyUpstreamOverwrite to accept options with locale.
  - Send locale via query/body accordingly.

Backend (Go)
- controller/model_sync.go
  - Accept locale from query/body and resolve i18n upstream URLs.
  - Add SYNC_UPSTREAM_BASE for upstream base override (default: https://basellm.github.io/llm-metadata).
  - Make HTTP timeouts/retries/limits configurable:
    - SYNC_HTTP_TIMEOUT_SECONDS, SYNC_HTTP_RETRY, SYNC_HTTP_MAX_MB
  - Add ETag-based caching and support both envelope and pure array JSON formats.
  - Concurrently fetch vendors and models; improve error responses with locale and source URLs.
  - Include source meta (locale, models_url, vendors_url) in success payloads.

Notes
- No breaking changes expected.
- Lint passes for touched files.

web/src/i18n/locales/en.json

@@ -1801,7 +1801,7 @@
   "已绑定渠道": "Bound channels",
   "更新时间": "Update time",
   "未配置模型": "No model configured",
-  "预填组管理": "Pre-filled group management",
+  "预填组管理": "Pre-filled group",
   "搜索供应商": "Search vendor",
   "新增供应商": "Add vendor",
   "创建新的模型": "Create new model",
@@ -2057,9 +2057,20 @@
   "侧边栏设置保存成功": "Sidebar settings saved successfully",
   "需要登录访问": "Require Login",
   "开启后未登录用户无法访问模型广场": "When enabled, unauthenticated users cannot access the model marketplace",
-  "同步官方": "Sync official",
   "参与官方同步": "Participate in official sync",
   "关闭后，此模型将不会被“同步官方”自动覆盖或创建": "When turned off, this model will be skipped by Sync official (no auto create/overwrite)",
+  "同步": "Sync",
+  "同步向导": "Sync Wizard",
+  "选择方式": "Select method",
+  "选择同步来源": "Select sync source",
+  "选择语言": "Select language",
+  "选择同步语言": "Select sync language",
+  "请选择同步语言": "Please select sync language",
+  "从官方模型库同步": "Sync from official model library",
+  "官方模型同步": "Official models sync",
+  "从配置文件同步": "Sync from config file",
+  "配置文件同步": "Config file sync",
+  "开始同步": "Start sync",
   "选择要覆盖的冲突项": "Select conflict items to overwrite",
   "点击查看差异": "Click to view differences",
   "无冲突项": "No conflict items",