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/index.html

@@ -18,6 +18,7 @@
     <link rel="stylesheet" href="/src/css/terminal.css">
     <link rel="stylesheet" href="/src/css/splitpane.css">
     <link rel="stylesheet" href="/src/css/recordings.css">
+    <link rel="stylesheet" href="/src/css/sftp.css">
     <link rel="stylesheet" href="/src/css/forms.css">
     <link rel="stylesheet" href="/src/css/modals.css">
     <link rel="stylesheet" href="/src/css/github.css">
@@ -198,6 +199,7 @@
     <script src="/src/js/commands.js"></script>
     <script src="/src/js/secrets.js"></script>
     <script src="/src/js/recordings.js"></script>
+    <script src="/src/js/sftp.js"></script>
     <script src="/src/js/splitpane.js"></script>
     <script src="/src/js/app.js"></script>
 </body>