refactor(config): remove v1 auto-migration and improve error handling

BREAKING CHANGE: Runtime auto-migration from v1 to v2 config format has been removed.

Changes:
- Remove automatic v1→v2 migration logic from MultiAppConfig::load()
- Improve v1 detection using structural analysis (checks for 'apps' key absence)
- Return clear error with migration instructions when v1 config is detected
- Add comprehensive tests for config loading edge cases
- Fix false positive detection when v1 config contains 'version' or 'mcp' fields

Migration path for users:
1. Install v3.2.x to perform one-time auto-migration, OR
2. Manually edit ~/.cc-switch/config.json to v2 format

Rationale:
- Separates concerns: load() should be read-only, not perform side effects
- Fail-fast principle: unsupported formats should error immediately
- Simplifies code maintenance by removing migration logic from hot path

Tests added:
- load_v1_config_returns_error_and_does_not_write
- load_v1_with_extra_version_still_treated_as_v1
- load_invalid_json_returns_parse_error_and_does_not_write
- load_valid_v2_config_succeeds

src-tauri/tests/app_config_load.rs

@@ -0,0 +1,105 @@
+use std::fs;
+use std::path::PathBuf;
+
+use cc_switch_lib::{AppError, MultiAppConfig};
+
+mod support;
+use support::{ensure_test_home, reset_test_fs, test_mutex};
+
+fn cfg_path() -> PathBuf {
+    let home = std::env::var("HOME").expect("HOME should be set by ensure_test_home");
+    PathBuf::from(home).join(".cc-switch").join("config.json")
+}
+
+#[test]
+fn load_v1_config_returns_error_and_does_not_write() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let home = ensure_test_home();
+    let path = cfg_path();
+    fs::create_dir_all(path.parent().unwrap()).expect("create cfg dir");
+
+    // 最小 v1 形状：providers + current，且不含 version/apps/mcp
+    let v1_json = r#"{"providers":{},"current":""}"#;
+    fs::write(&path, v1_json).expect("seed v1 json");
+    let before = fs::read_to_string(&path).expect("read before");
+
+    let err = MultiAppConfig::load().expect_err("v1 should not be auto-migrated");
+    match err {
+        AppError::Localized { key, .. } => assert_eq!(key, "config.unsupported_v1"),
+        other => panic!("expected Localized v1 error, got {other:?}"),
+    }
+
+    // 文件不应有任何变化，且不应生成 .bak
+    let after = fs::read_to_string(&path).expect("read after");
+    assert_eq!(before, after, "config.json should not be modified");
+    let bak = home.join(".cc-switch").join("config.json.bak");
+    assert!(!bak.exists(), ".bak should not be created on load error");
+}
+
+#[test]
+fn load_v1_with_extra_version_still_treated_as_v1() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let home = ensure_test_home();
+    let path = cfg_path();
+    std::fs::create_dir_all(path.parent().unwrap()).expect("create cfg dir");
+
+    // 畸形：包含 providers + current + version，但没有 apps，应按 v1 处理
+    let v1_like = r#"{"providers":{},"current":"","version":2}"#;
+    std::fs::write(&path, v1_like).expect("seed v1-like json");
+    let before = std::fs::read_to_string(&path).expect("read before");
+
+    let err = MultiAppConfig::load().expect_err("v1-like should not be parsed as v2");
+    match err {
+        AppError::Localized { key, .. } => assert_eq!(key, "config.unsupported_v1"),
+        other => panic!("expected Localized v1 error, got {other:?}"),
+    }
+
+    let after = std::fs::read_to_string(&path).expect("read after");
+    assert_eq!(before, after, "config.json should not be modified");
+    let bak = home.join(".cc-switch").join("config.json.bak");
+    assert!(!bak.exists(), ".bak should not be created on v1-like error");
+}
+
+#[test]
+fn load_invalid_json_returns_parse_error_and_does_not_write() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let home = ensure_test_home();
+    let path = cfg_path();
+    fs::create_dir_all(path.parent().unwrap()).expect("create cfg dir");
+
+    fs::write(&path, "{not json").expect("seed invalid json");
+    let before = fs::read_to_string(&path).expect("read before");
+
+    let err = MultiAppConfig::load().expect_err("invalid json should error");
+    match err {
+        AppError::Json { .. } => {}
+        other => panic!("expected Json error, got {other:?}"),
+    }
+
+    let after = fs::read_to_string(&path).expect("read after");
+    assert_eq!(before, after, "config.json should remain unchanged");
+    let bak = home.join(".cc-switch").join("config.json.bak");
+    assert!(!bak.exists(), ".bak should not be created on parse error");
+}
+
+#[test]
+fn load_valid_v2_config_succeeds() {
+    let _guard = test_mutex().lock().expect("acquire test mutex");
+    reset_test_fs();
+    let _home = ensure_test_home();
+    let path = cfg_path();
+    fs::create_dir_all(path.parent().unwrap()).expect("create cfg dir");
+
+    // 使用默认结构序列化为 v2
+    let default_cfg = MultiAppConfig::default();
+    let json = serde_json::to_string_pretty(&default_cfg).expect("serialize default cfg");
+    fs::write(&path, json).expect("write v2 json");
+
+    let loaded = MultiAppConfig::load().expect("v2 should load successfully");
+    assert_eq!(loaded.version, 2);
+    assert!(loaded.get_manager(&cc_switch_lib::AppType::Claude).is_some());
+    assert!(loaded.get_manager(&cc_switch_lib::AppType::Codex).is_some());
+}