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/Cargo.lock

@@ -291,6 +291,17 @@ version = "1.1.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1505bd5d3d116872e7271a6d4e16d81d0c8570876c8de68093a09ac269d8aac0"
 
+[[package]]
+name = "auto-launch"
+version = "0.5.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1f012b8cc0c850f34117ec8252a44418f2e34a2cf501de89e29b241ae5f79471"
+dependencies = [
+ "dirs 4.0.0",
+ "thiserror 1.0.69",
+ "winreg 0.10.1",
+]
+
 [[package]]
 name = "autocfg"
 version = "1.5.0"
@@ -598,6 +609,7 @@ name = "cc-switch"
 version = "3.7.0"
 dependencies = [
  "anyhow",
+ "auto-launch",
  "chrono",
  "dirs 5.0.1",
  "futures",
@@ -982,6 +994,15 @@ dependencies = [
  "subtle",
 ]
 
+[[package]]
+name = "dirs"
+version = "4.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ca3aa72a6f96ea37bbc5aa912f6788242832f75369bdfdadcb0e38423f100059"
+dependencies = [
+ "dirs-sys 0.3.7",
+]
+
 [[package]]
 name = "dirs"
 version = "5.0.1"
@@ -1000,6 +1021,17 @@ dependencies = [
  "dirs-sys 0.5.0",
 ]
 
+[[package]]
+name = "dirs-sys"
+version = "0.3.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1b1d1d91c932ef41c0f2663aa8b0ca0342d444d842c06914aa0a7e352d0bada6"
+dependencies = [
+ "libc",
+ "redox_users 0.4.6",
+ "winapi",
+]
+
 [[package]]
 name = "dirs-sys"
 version = "0.4.1"
@@ -6397,6 +6429,15 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "winreg"
+version = "0.10.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "80d0f4e272c85def139476380b12f9ac60926689dd2e01d4923222f40580869d"
+dependencies = [
+ "winapi",
+]
+
 [[package]]
 name = "winreg"
 version = "0.52.0"