feat: add SFTP drag-and-drop file handling and upload functionality

- Implemented drag-and-drop support for file uploads in the terminal.
- Added a modal for users to choose between uploading files or pasting contents.
- Enhanced SFTP browser with a directory picker for selecting upload destinations.
- Introduced methods for handling file uploads and reading file contents from the local system.
- Added a new API endpoint for uploading raw file data to the server.
- Implemented session management improvements to retrieve the current working directory for SSH sessions.
- Updated documentation to include an overview and quick start guide for Nexus Terminal.

docs/quick-start.md

@@ -0,0 +1,61 @@
+# Nexus Terminal Quick Start
+
+Nexus Terminal is a desktop client for SSH, Telnet, RAW TCP, and GitHub sessions. This page is a shareable quick start for everyday use.
+
+## Get connected in minutes
+
+1. Open Nexus Terminal.
+2. Click **New** in the sidebar.
+3. Choose a protocol and enter the connection details.
+4. Click **Connect** to open the session in a new tab.
+
+One-off session? Use **Cmd+Shift+Q** / **Ctrl+Shift+Q** for quick connect.
+
+## Daily workflow
+
+- **Tabs and splits**: Drag tabs to reorder, and split the workspace to view multiple sessions at once.
+- **Broadcast input**: Toggle with **Cmd+Shift+B** to send the same command to all connected sessions.
+- **Snippets**: Save common commands and run them with one click.
+- **Search**: Press **Cmd+F** to search within the active terminal.
+
+## File transfers on SSH
+
+Click the **folder** icon on an SSH tab to open the SFTP browser. You can browse, upload, download, rename, and delete files.
+
+### Drag and drop files
+
+Drop file(s) into the terminal and choose to:
+
+- **Upload** to a selected remote folder, or
+- **Paste contents** into the terminal as if typed.
+
+The upload picker lists folders only and marks permissions:
+
+- **READ ONLY**: you can read but cannot write
+- **NO READ**: you cannot access the folder
+
+## Security basics
+
+- Credentials are stored in the OS keychain where available.
+- An encrypted vault is used as a fallback.
+- SSH host key checks can be set per profile (Strict, Ask, Accept new, Insecure).
+
+## Common shortcuts
+
+| Shortcut          | Action                      |
+| ----------------- | --------------------------- |
+| Cmd + K           | Command palette             |
+| Cmd + N           | New connection              |
+| Cmd + W           | Close tab                   |
+| Cmd + F           | Find in terminal            |
+| Cmd + Shift + S   | Snippet library             |
+| Cmd + Shift + B   | Broadcast input             |
+| Cmd + Shift + Q   | Quick connect               |
+| Cmd + Shift + R   | Toggle session recording    |
+| Cmd + Shift + D   | Split pane vertically       |
+| Cmd + Shift + E   | Split pane horizontally     |
+
+## Need more detail?
+
+- Product overview: [docs/overview.md](overview.md)
+- User manual: [docs/user-manual.md](user-manual.md)