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.js

@@ -2,6 +2,10 @@
 // Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
 // This file is automatically generated. DO NOT EDIT
 
+export function CheckForUpdates() {
+  return window['go']['api']['Service']['CheckForUpdates']();
+}
+
 export function ClearSessionHistory(arg1) {
   return window['go']['api']['Service']['ClearSessionHistory'](arg1);
 }
@@ -82,6 +86,10 @@ export function ImportProfilesEncrypted(arg1, arg2) {
   return window['go']['api']['Service']['ImportProfilesEncrypted'](arg1, arg2);
 }
 
+export function ImportSSHConfig(arg1, arg2) {
+  return window['go']['api']['Service']['ImportSSHConfig'](arg1, arg2);
+}
+
 export function InitialiseSecretStore(arg1) {
   return window['go']['api']['Service']['InitialiseSecretStore'](arg1);
 }
@@ -170,6 +178,34 @@ export function ResizeSession(arg1, arg2, arg3) {
   return window['go']['api']['Service']['ResizeSession'](arg1, arg2, arg3);
 }
 
+export function SFTPDownload(arg1, arg2) {
+  return window['go']['api']['Service']['SFTPDownload'](arg1, arg2);
+}
+
+export function SFTPList(arg1, arg2) {
+  return window['go']['api']['Service']['SFTPList'](arg1, arg2);
+}
+
+export function SFTPMkdir(arg1, arg2) {
+  return window['go']['api']['Service']['SFTPMkdir'](arg1, arg2);
+}
+
+export function SFTPPwd(arg1) {
+  return window['go']['api']['Service']['SFTPPwd'](arg1);
+}
+
+export function SFTPRemove(arg1, arg2) {
+  return window['go']['api']['Service']['SFTPRemove'](arg1, arg2);
+}
+
+export function SFTPRename(arg1, arg2, arg3) {
+  return window['go']['api']['Service']['SFTPRename'](arg1, arg2, arg3);
+}
+
+export function SFTPUpload(arg1, arg2) {
+  return window['go']['api']['Service']['SFTPUpload'](arg1, arg2);
+}
+
 export function SaveFile(arg1, arg2) {
   return window['go']['api']['Service']['SaveFile'](arg1, arg2);
 }