chore: add GUI quickstart docs and update README for 0.1.0-beta.0

docs/gui-quickstart-linux.md

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+# Konduit GUI — Linux Quickstart
+
+## 1. Download
+
+Download the `Konduit-*-x86_64.AppImage` from the [Releases](../../releases) page.
+
+## 2. Make it executable
+
+```bash
+chmod +x Konduit-*-x86_64.AppImage
+```
+
+## 3. Run
+
+Double-click the AppImage in your file manager, or launch it from the terminal:
+
+```bash
+./Konduit-*-x86_64.AppImage
+```
+
+No installation required. The AppImage is self-contained.
+
+## 4. Load your config
+
+Your server administrator will provide a `client.toml` config file. On first launch, tap **Open config** and select the file.
+
+## 5. Privileges
+
+Konduit creates a TUN network device, which requires elevated network capability. The AppImage requests this automatically via a `pkexec` prompt on first run. If you see a password prompt, enter your user password to grant the capability.
+
+If `pkexec` is not available on your system (some minimal desktop environments), grant the capability manually once:
+
+```bash
+sudo setcap cap_net_admin=+ep ~/.local/share/konduit/konduit-helper
+```
+
+## 6. Autostart on login (optional)
+
+To start Konduit automatically when you log in, enable autostart in the app: **Settings → Start on login**.
+
+Alternatively, copy the provided desktop entry to your autostart directory:
+
+```bash
+cp ~/.local/share/applications/konduit.desktop ~/.config/autostart/
+```
+
+## Troubleshooting
+
+**AppImage won't launch** — your system may be missing FUSE. Install it:
+
+```bash
+# Debian / Ubuntu
+sudo apt install libfuse2
+
+# Fedora / RHEL
+sudo dnf install fuse
+```
+
+**"Failed to create TUN device"** — the capability was not granted. Run the `setcap` command from step 5 and relaunch.
+
+**Config not found on relaunch** — the app remembers the last config path. If you move the file, use **Open config** again to reselect it.