feat: add proxy configuration support

- Add proxy settings UI component with enable/disable toggle
  - Support HTTP, HTTPS, NO_PROXY, and ALL_PROXY environment variables
  - Store proxy settings in app database for persistence
  - Apply proxy settings on app startup and when saved
  - Pass proxy environment variables to Claude command execution
  - Integrate proxy settings into main Settings page with unified save
  - Add proxy support for both system binary and sidecar execution

  This allows users to configure proxy settings for Claude API requests,
  which is essential for users behind corporate firewalls or in regions
  requiring proxy access.

  Fixes network connectivity issues in restricted environments.

src-tauri/src/commands/mod.rs

@@ -4,3 +4,4 @@ pub mod mcp;
 pub mod usage;
 pub mod storage;
 pub mod slash_commands;
+pub mod proxy;