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

go.mod

@@ -1,14 +1,15 @@
 module github.com/copilot-ssh-client/app
 
-go 1.22.0
+go 1.23.0
 
 require (
 	github.com/google/uuid v1.6.0
 	github.com/mattn/go-sqlite3 v1.14.22
+	github.com/pkg/sftp v1.13.10
 	github.com/wailsapp/wails/v2 v2.12.0
 	github.com/zalando/go-keyring v0.2.5
-	golang.org/x/crypto v0.33.0
-	golang.org/x/text v0.22.0
+	golang.org/x/crypto v0.41.0
+	golang.org/x/text v0.28.0
 )
 
 require (
@@ -20,6 +21,7 @@ require (
 	github.com/godbus/dbus/v5 v5.1.0 // indirect
 	github.com/gorilla/websocket v1.5.3 // indirect
 	github.com/jchv/go-winloader v0.0.0-20210711035445-715c2860da7e // indirect
+	github.com/kr/fs v0.1.0 // indirect
 	github.com/labstack/echo/v4 v4.13.3 // indirect
 	github.com/labstack/gommon v0.4.2 // indirect
 	github.com/leaanthony/go-ansi-parser v1.6.1 // indirect
@@ -37,6 +39,6 @@ require (
 	github.com/valyala/fasttemplate v1.2.2 // indirect
 	github.com/wailsapp/go-webview2 v1.0.22 // indirect
 	github.com/wailsapp/mimetype v1.4.1 // indirect
-	golang.org/x/net v0.35.0 // indirect
-	golang.org/x/sys v0.30.0 // indirect
+	golang.org/x/net v0.42.0 // indirect
+	golang.org/x/sys v0.35.0 // indirect
 )