feat(backend): add auto-launch functionality

Implement system auto-launch feature to allow CC-Switch to start
automatically on system boot, improving user convenience.

Backend Implementation:
- auto_launch.rs: New module for auto-launch management
  * Cross-platform support using auto-launch crate
  * Enable/disable auto-launch with system integration
  * Proper error handling for permission issues
  * Platform-specific implementations (macOS/Windows/Linux)

Command Layer:
- Add get_auto_launch command to check current status
- Add set_auto_launch command to toggle auto-start
- Integrate commands with settings API

Settings Integration:
- Extend Settings struct with auto_launch field
- Persist auto-launch preference in settings store
- Automatic state synchronization on app startup

Dependencies:
- Add auto-launch ^0.5.0 to Cargo.toml
- Update Cargo.lock with new dependency tree

Technical Details:
- Uses platform-specific auto-launch mechanisms:
  * macOS: Login Items via LaunchServices
  * Windows: Registry Run key
  * Linux: XDG autostart desktop files
- Handles edge cases like permission denials gracefully
- Maintains settings consistency across app restarts

This feature enables users to have CC-Switch readily available
after system boot without manual intervention, particularly useful
for users who frequently switch between API providers.

src-tauri/src/lib.rs

@@ -1,5 +1,6 @@
 mod app_config;
 mod app_store;
+mod auto_launch;
 mod claude_mcp;
 mod claude_plugin;
 mod codex_config;
@@ -717,6 +718,9 @@ pub fn run() {
             commands::get_skill_repos,
             commands::add_skill_repo,
             commands::remove_skill_repo,
+            // Auto launch
+            commands::set_auto_launch,
+            commands::get_auto_launch_status,
         ]);
 
     let app = builder