Phase 4: SFTP file browser, SSH config import, and update checking

SFTP file browser:
- Created internal/sftpclient wrapping github.com/pkg/sftp for remote file
  operations (list, stat, mkdir, remove, rename, download, upload, pwd)
- Integrated SFTP into session manager with lazy client creation per session
- Added API methods for all SFTP operations with native file dialogs
- Built SFTPBrowser frontend component — right-side resizable panel with
  breadcrumb navigation, sorted file listing, context actions (download,
  rename, delete), upload, new folder, and custom modal dialogs
- Toggle via Cmd+Shift+F on active SSH sessions
- Auto-refits terminal when panel opens/closes/resizes
- Panel cleaned up automatically when session closes

OpenSSH config import:
- Created internal/sshconfig parser for ~/.ssh/config Host blocks
- Converts to profiles with hostname, port, user, identity file,
  ProxyJump (first hop), ForwardAgent, and Compression support
- Skips wildcard-only hosts and Match blocks
- Import button added to Settings > About tab

Automatic update checking:
- Created internal/updater querying GitHub Releases API
- Simple semver comparison, 10-second timeout
- Dev builds never trigger update prompts
- Check button added to Settings > About tab

frontend/wailsjs/go/api/Service.d.ts

@@ -3,6 +3,8 @@
 import {models} from '../models';
 import {context} from '../models';
 
+export function CheckForUpdates():Promise<Record<string, any>>;
+
 export function ClearSessionHistory(arg1:string):Promise<void>;
 
 export function CloseAllSessions():Promise<void>;
@@ -43,6 +45,8 @@ export function ImportProfiles(arg1:string,arg2:string):Promise<number>;
 
 export function ImportProfilesEncrypted(arg1:string,arg2:string):Promise<number>;
 
+export function ImportSSHConfig(arg1:string,arg2:string):Promise<number>;
+
 export function InitialiseSecretStore(arg1:string):Promise<void>;
 
 export function IsRecording(arg1:string):Promise<boolean>;
@@ -87,6 +91,20 @@ export function RenameSession(arg1:string,arg2:string):Promise<void>;
 
 export function ResizeSession(arg1:string,arg2:number,arg3:number):Promise<void>;
 
+export function SFTPDownload(arg1:string,arg2:string):Promise<void>;
+
+export function SFTPList(arg1:string,arg2:string):Promise<Array<Record<string, any>>>;
+
+export function SFTPMkdir(arg1:string,arg2:string):Promise<void>;
+
+export function SFTPPwd(arg1:string):Promise<string>;
+
+export function SFTPRemove(arg1:string,arg2:string):Promise<void>;
+
+export function SFTPRename(arg1:string,arg2:string,arg3:string):Promise<void>;
+
+export function SFTPUpload(arg1:string,arg2:string):Promise<void>;
+
 export function SaveFile(arg1:string,arg2:string):Promise<string>;
 
 export function SavePreferences(arg1:models.AppPreferences):Promise<void>;