feat(providers): seed an official preset on startup for Claude/Codex/Gemini

New and existing users now see a built-in "Claude Official" / "OpenAI
Official" / "Google Official" entry in their provider list, so switching
back to the official endpoint is one click away instead of buried in the
README.

- New providers_seed.rs holds the three seeds (id, name, settings_config,
  icon) keyed by AppType, with a single is_official_seed_id() helper that
  scans OFFICIAL_SEEDS so the id list has one source of truth.
- Database::init_default_official_providers() runs once per database
  (gated by an official_providers_seeded setting flag), appends each seed
  to the end of the sort order, and never touches is_current.
- Startup also auto-imports the live config (settings.json / auth.json /
  .env) as a "default" provider before seeding, so users with an existing
  manual config don't lose it when they click the official preset.
- Database::has_non_official_seed_provider() replaces the get_all_providers
  call in import_default_config's gating check with an id-only scan,
  dropping the N+1 endpoint sub-queries from every startup.

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

@@ -999,11 +999,12 @@ pub fn import_default_config(state: &AppState, app_type: AppType) -> Result<bool
         return Ok(false);
     }
 
-    {
-        let providers = state.db.get_all_providers(app_type.as_str())?;
-        if !providers.is_empty() {
-            return Ok(false); // 已有供应商，跳过
-        }
+    // 允许 "只有官方 seed 预设" 的情况下继续导入 live：
+    // - 启动编排顺序是先 import 后 seed，新用户启动时 providers 为空，导入照常
+    // - 老用户已有非 seed provider，跳过导入（正确）
+    // - 用户手动点 ProviderEmptyState 的导入按钮时，与官方 seed 共存而不被阻塞
+    if state.db.has_non_official_seed_provider(app_type.as_str())? {
+        return Ok(false);
     }
 
     let settings_config = match app_type {