[Feature]: feat(desktop): add Linux AppImage build for x86_64

### Is your feature request related to a problem?

The desktop application currently provides builds for macOS (`.zip`) and Windows (`.exe`), but there is no native Linux desktop release. QwenPaw uses **Tauri** under the hood, which natively supports Linux packaging including AppImage — so this would be a straightforward addition to the existing build pipeline.

Many Linux users (including myself) would benefit from a simple, portable `.AppImage` binary for `x86_64` that can be downloaded and run without requiring system dependencies or a build toolchain.

### Describe the solution you'd like

Add a Linux AppImage build target to the Tauri desktop packaging for `x86_64`:

- **Format:** AppImage (`.AppImage`)
- **Architecture:** `x86_64`
- **Included in the same release** as macOS and Windows assets (e.g., `QwenPaw-{version}-linux-x86_64.AppImage`)

Since Tauri already supports Linux and the current CI builds are Tauri-based, this should mainly require adding the Linux platform to the Tauri build configuration (`tauri.conf.json` → `bundle.linux`) and adding a Linux target to the release CI workflow.

### Describe alternatives you've considered

- **Flatpak / Snap:** Would require additional packaging and distribution setup. AppImage is simpler for a single-file portable binary.
- **Pre-built binary (`.tar.gz`):** Users would need to manage desktop integration manually. AppImage handles this out of the box.
- **Building from source:** Possible but requires Rust + Node.js + Tauri dependencies, which is a significant barrier for end users.

### Additional context

- Tauri has first-class Linux support: https://tauri.app/distribute/
- The current CI already publishes macOS and Windows builds via Tauri, so the pipeline is in place.
- The latest release (`v1.1.12.post2`) assets:
- `QwenPaw-1.1.12.post2-macOS.zip`
- `QwenPaw-Setup-1.1.12.post2.exe`
- `QwenPaw-Tauri-1.1.12.post2-macOS.zip`
- `QwenPaw-Tauri-1.1.12.post2-Windows-setup.exe`
- **Missing:** `QwenPaw-1.1.12.post2-linux-x86_64.AppImage`