fix: prevent common config loss during proxy takeover and stabilize snippet lifecycle

- Make sync_current_provider_for_app takeover-aware: update restore
  backup instead of overwriting live config when proxy is active
- Introduce explicit "cleared" flag for common config snippets to
  prevent auto-extraction from resurrecting user-cleared snippets
- Reorder startup: extract snippets from clean live files before
  restoring proxy takeover state
- Add one-time migration flag to skip legacy commonConfigEnabled
  migration on subsequent startups
- Add regression tests for takeover backup preservation, explicit
  clear semantics, and migration flag roundtrip

src-tauri/src/commands/config.rs

@@ -212,20 +212,22 @@ pub async fn set_claude_common_config_snippet(
     snippet: String,
     state: tauri::State<'_, crate::store::AppState>,
 ) -> Result<(), String> {
+    let is_cleared = snippet.trim().is_empty();
+
     if !snippet.trim().is_empty() {
         serde_json::from_str::<serde_json::Value>(&snippet).map_err(invalid_json_format_error)?;
     }
 
-    let value = if snippet.trim().is_empty() {
-        None
-    } else {
-        Some(snippet)
-    };
+    let value = if is_cleared { None } else { Some(snippet) };
 
     state
         .db
         .set_config_snippet("claude", value)
         .map_err(|e| e.to_string())?;
+    state
+        .db
+        .set_config_snippet_cleared("claude", is_cleared)
+        .map_err(|e| e.to_string())?;
     Ok(())
 }
 
@@ -246,6 +248,7 @@ pub async fn set_common_config_snippet(
     snippet: String,
     state: tauri::State<'_, crate::store::AppState>,
 ) -> Result<(), String> {
+    let is_cleared = snippet.trim().is_empty();
     let old_snippet = state
         .db
         .get_config_snippet(&app_type)
@@ -253,11 +256,7 @@ pub async fn set_common_config_snippet(
 
     validate_common_config_snippet(&app_type, &snippet)?;
 
-    let value = if snippet.trim().is_empty() {
-        None
-    } else {
-        Some(snippet)
-    };
+    let value = if is_cleared { None } else { Some(snippet) };
 
     if matches!(app_type.as_str(), "claude" | "codex" | "gemini") {
         if let Some(legacy_snippet) = old_snippet
@@ -278,6 +277,10 @@ pub async fn set_common_config_snippet(
         .db
         .set_config_snippet(&app_type, value)
         .map_err(|e| e.to_string())?;
+    state
+        .db
+        .set_config_snippet_cleared(&app_type, is_cleared)
+        .map_err(|e| e.to_string())?;
 
     if matches!(app_type.as_str(), "claude" | "codex" | "gemini") {
         let app = AppType::from_str(&app_type).map_err(|e| e.to_string())?;

src-tauri/src/commands/provider.rs

@@ -103,16 +103,16 @@ fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result
         // 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()
+            .should_auto_extract_config_snippet(app_type.as_str())?
         {
             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));
+                    let _ = state
+                        .db
+                        .set_config_snippet_cleared(app_type.as_str(), false);
                 }
                 _ => {}
             }