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

internal/sshclient/sshclient.go

@@ -304,6 +304,14 @@ func (c *Client) Forwards() *ForwardManager {
 	return c.forwards
 }
 
+// SSHConn returns the underlying ssh.Client for use by subsystems such as
+// SFTP. Returns nil if not connected.
+func (c *Client) SSHConn() *ssh.Client {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	return c.sshClient
+}
+
 // buildAuthMethods constructs the SSH authentication method list from the
 // profile's SSHConfig and the provided credential callbacks.
 func (c *Client) buildAuthMethods(sc models.SSHConfig, getPassword func() (string, error), getPassphrase func() (string, error)) ([]ssh.AuthMethod, error) {