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/process/registry.rs

@@ -83,6 +83,41 @@ impl ProcessRegistry {
         self.register_process_internal(run_id, process_info, child)
     }
 
+    /// Register a new running agent process using sidecar (similar to register_process but for sidecar children)
+    pub fn register_sidecar_process(
+        &self,
+        run_id: i64,
+        agent_id: i64,
+        agent_name: String,
+        pid: u32,
+        project_path: String,
+        task: String,
+        model: String,
+        child: tauri_plugin_shell::process::Child,
+    ) -> Result<(), String> {
+        let process_info = ProcessInfo {
+            run_id,
+            process_type: ProcessType::AgentRun { agent_id, agent_name },
+            pid,
+            started_at: Utc::now(),
+            project_path,
+            task,
+            model,
+        };
+
+        // For sidecar processes, we register without the child handle since it's managed differently
+        let mut processes = self.processes.lock().map_err(|e| e.to_string())?;
+        
+        let process_handle = ProcessHandle {
+            info: process_info,
+            child: Arc::new(Mutex::new(None)), // No tokio::process::Child handle for sidecar
+            live_output: Arc::new(Mutex::new(String::new())),
+        };
+
+        processes.insert(run_id, process_handle);
+        Ok(())
+    }
+
     /// Register a new Claude session (without child process - handled separately)
     pub fn register_claude_session(
         &self,