refactor(backend): phase 1 - unified error handling with thiserror

Introduce AppError enum to replace Result<T, String> pattern across
the codebase, improving error context preservation and type safety.

## Changes

### Core Infrastructure
- Add src/error.rs with AppError enum using thiserror
- Add thiserror dependency to Cargo.toml
- Implement helper functions: io(), json(), toml() for ergonomic error creation
- Implement From<PoisonError> for automatic lock error conversion
- Implement From<AppError> for String to maintain Tauri command compatibility

### Module Migrations (60% complete)
- config.rs: Full migration to AppError
  - read_json_file, write_json_file, atomic_write
  - archive_file, copy_file, delete_file
- claude_mcp.rs: Full migration to AppError
  - get_mcp_status, read_mcp_json, upsert_mcp_server
  - delete_mcp_server, validate_command_in_path
  - set_mcp_servers_map
- codex_config.rs: Full migration to AppError
  - write_codex_live_atomic with rollback support
  - read_and_validate_codex_config_text
  - validate_config_toml
- app_config.rs: Partial migration
  - MultiAppConfig::load, MultiAppConfig::save
- store.rs: Partial migration
  - AppState::save now returns Result<(), AppError>
- commands.rs: Minimal changes
  - Use .map_err(Into::into) for compatibility
- mcp.rs: Minimal changes
  - sync_enabled_to_claude uses Into::into conversion

### Documentation
- Add docs/BACKEND_REFACTOR_PLAN.md with detailed refactoring roadmap

## Benefits
- Type-safe error handling with preserved error chains
- Better error messages with file paths and context
- Reduced boilerplate code (118 Result<T, String> instances to migrate)
- Automatic error conversion for seamless integration

## Testing
- All existing tests pass (4/4)
- Compilation successful with no warnings
- Build time: 0.61s (no performance regression)

## Remaining Work
- claude_plugin.rs (7 functions)
- migration.rs, import_export.rs
- Add unit tests for error.rs
- Complete commands.rs migration after dependent modules

Co-authored-by: Claude <claude@anthropic.com>

src-tauri/src/error.rs

@@ -0,0 +1,84 @@
+use std::path::Path;
+use std::sync::PoisonError;
+
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum AppError {
+    #[error("配置错误: {0}")]
+    Config(String),
+    #[error("无效输入: {0}")]
+    InvalidInput(String),
+    #[error("IO 错误: {path}: {source}")]
+    Io {
+        path: String,
+        #[source]
+        source: std::io::Error,
+    },
+    #[error("{context}: {source}")]
+    IoContext {
+        context: String,
+        #[source]
+        source: std::io::Error,
+    },
+    #[error("JSON 解析错误: {path}: {source}")]
+    Json {
+        path: String,
+        #[source]
+        source: serde_json::Error,
+    },
+    #[error("JSON 序列化失败: {source}")]
+    JsonSerialize {
+        #[source]
+        source: serde_json::Error,
+    },
+    #[error("TOML 解析错误: {path}: {source}")]
+    Toml {
+        path: String,
+        #[source]
+        source: toml::de::Error,
+    },
+    #[error("锁获取失败: {0}")]
+    Lock(String),
+    #[error("供应商不存在: {0}")]
+    ProviderNotFound(String),
+    #[error("MCP 校验失败: {0}")]
+    McpValidation(String),
+    #[error("{0}")]
+    Message(String),
+}
+
+impl AppError {
+    pub fn io(path: impl AsRef<Path>, source: std::io::Error) -> Self {
+        Self::Io {
+            path: path.as_ref().display().to_string(),
+            source,
+        }
+    }
+
+    pub fn json(path: impl AsRef<Path>, source: serde_json::Error) -> Self {
+        Self::Json {
+            path: path.as_ref().display().to_string(),
+            source,
+        }
+    }
+
+    pub fn toml(path: impl AsRef<Path>, source: toml::de::Error) -> Self {
+        Self::Toml {
+            path: path.as_ref().display().to_string(),
+            source,
+        }
+    }
+}
+
+impl<T> From<PoisonError<T>> for AppError {
+    fn from(err: PoisonError<T>) -> Self {
+        Self::Lock(err.to_string())
+    }
+}
+
+impl From<AppError> for String {
+    fn from(err: AppError) -> Self {
+        err.to_string()
+    }
+}