fix(mcp): stop cross-app failures from blocking MCP re-projection

sync_all_enabled iterated AppType::all() with `?`, so one app's corrupt
live file (e.g. a broken ~/.claude.json, which passes the existence
gate but fails to parse) blocked every app behind it in the iteration
order -- and bubbled the error into whatever operation triggered the
sync:

- switch/save had just rewritten only the target app's live file, yet a
  broken unrelated file failed the whole operation after DB and live
  were already updated, reporting a false "switch failed" to the user.
  Both now project only the target app (sync_enabled_for_app) and
  degrade projection failure to a warning: the primary operation has
  already taken effect, and the projection self-heals on the next
  switch or MCP toggle.
- sync_current_provider_for_app_to_live syncs a single app; it now
  projects that app only, keeping failures (which can only concern the
  target app) as errors.
- sync_current_to_live (config import / cloud-sync restore) keeps the
  all-apps sweep, but sync_all_enabled is now best-effort: it projects
  every app, collects failures, and reports them aggregated. Its error
  is held until after skill sync so an MCP failure no longer skips it.

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

@@ -948,7 +948,10 @@ pub(crate) fn sync_current_provider_for_app_to_live(
         }
     }
 
-    McpService::sync_all_enabled(state)?;
+    // 本函数语义是"把这个应用同步到 live"，MCP 重投影也只针对该应用；
+    // 全量 sync_all_enabled 会把无关应用的 live 损坏牵连进来。投影失败
+    // 上抛（不降级）：这里没有已变更的 DB 状态需要保护，调用方重试即可。
+    McpService::sync_enabled_for_app(state, app_type)?;
 
     Ok(())
 }
@@ -1016,8 +1019,10 @@ pub fn sync_current_to_live(state: &AppState) -> Result<(), AppError> {
         }
     }
 
-    // MCP sync
-    McpService::sync_all_enabled(state)?;
+    // MCP sync（best-effort 逐应用投影，内部已聚合失败）。错误暂存到
+    // Skill 同步之后再返回：MCP 的失败不该跳过 Skill 同步，但调用方
+    //（配置导入 / 云同步恢复）需要知道结果不完整。
+    let mcp_result = McpService::sync_all_enabled(state);
 
     // Skill sync
     for app_type in AppType::all() {
@@ -1027,7 +1032,7 @@ pub fn sync_current_to_live(state: &AppState) -> Result<(), AppError> {
         }
     }
 
-    Ok(())
+    mcp_result
 }
 
 /// Read current live settings for an app type

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

@@ -2134,8 +2134,16 @@ impl ProviderService {
                 }
             } else {
                 write_live_with_common_config(state.db.as_ref(), &app_type, &provider)?;
-                // Sync MCP
-                McpService::sync_all_enabled(state)?;
+                // 重写 live 后只重投影本应用的 MCP：全量 sync_all_enabled 会把
+                // 无关应用的 live 损坏（如 ~/.claude.json 坏 JSON）牵连进保存
+                // 流程。走到这里 DB 与 live 都已按新配置落盘，保存事实上已
+                // 成功；投影失败降级为警告，避免制造"保存失败"假象（MCP
+                // 投影可自愈：下次切换 / 任一 MCP 启停都会重新投影）。
+                if let Err(err) = McpService::sync_enabled_for_app(state, &app_type) {
+                    log::warn!(
+                        "保存供应商后重投影 {app_type:?} MCP 失败（将在下次同步时自愈）: {err}"
+                    );
+                }
             }
         }
 
@@ -2509,8 +2517,16 @@ impl ProviderService {
             }
         }
 
-        // Sync MCP
-        McpService::sync_all_enabled(state)?;
+        // 切换重写了目标应用的 live，只重投影该应用的 MCP（Codex 的
+        // [mcp_servers] 与 live 同文件，整体替换后必须补回；其余应用的
+        // MCP 文件独立于 live，投影是幂等维护）。不用全量 sync_all_enabled：
+        // 无关应用的 live 损坏（如 ~/.claude.json 坏 JSON）不该阻断切换。
+        // 走到这里 DB is_current 与 live 都已落盘，切换事实上已成功；
+        // 投影失败上抛会让前端报"切换失败"制造分裂假象，故降级为警告
+        // （MCP 投影可自愈：下次切换 / 任一 MCP 启停都会重新投影）。
+        if let Err(err) = McpService::sync_enabled_for_app(state, &app_type) {
+            log::warn!("切换供应商后重投影 {app_type:?} MCP 失败（将在下次同步时自愈）: {err}");
+        }
 
         Ok(result)
     }