refactor(provider): replace startup auto-import with manual import button

Remove the startup loop in lib.rs that auto-imported default providers
for Claude/Codex/Gemini. Move config snippet extraction logic into the
import_default_config command so it works when triggered manually.

Add an "Import Current Config" button to ProviderEmptyState, wired via
useMutation in ProviderList (shown only for standard apps). Update i18n
keys (zh/en/ja) with new button labels and revised empty state text.

src-tauri/src/commands/provider.rs

@@ -93,7 +93,29 @@ pub fn switch_provider(
 }
 
 fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result<bool, AppError> {
-    ProviderService::import_default_config(state, app_type)
+    let imported = ProviderService::import_default_config(state, app_type.clone())?;
+
+    if imported {
+        // Extract common config snippet (mirrors old startup logic in lib.rs)
+        if state
+            .db
+            .get_config_snippet(app_type.as_str())
+            .ok()
+            .flatten()
+            .is_none()
+        {
+            match ProviderService::extract_common_config_snippet(state, app_type.clone()) {
+                Ok(snippet) if !snippet.is_empty() && snippet != "{}" => {
+                    let _ = state
+                        .db
+                        .set_config_snippet(app_type.as_str(), Some(snippet));
+                }
+                _ => {}
+            }
+        }
+    }
+
+    Ok(imported)
 }
 
 #[cfg_attr(not(feature = "test-hooks"), doc(hidden))]