Creator: support streaming from Windows

**Title:** [Feature Request] Window/Application Capture Source (Support for Steam Link / Remote Play)

**Description:**
I am setting up a dual-PC streaming workflow using an M4 Mac Mini as the dedicated streaming machine. Instead of using a hardware capture card, I am using **Steam Link** (or Moonlight) to stream gameplay from my Windows PC to the Mac desktop.

To make Arkavo Creator a viable all-in-one broadcasting solution for this setup, I request the ability to use a specific **Application Window** as a video source.

### The Need
Currently, I understand the app may support camera inputs, but for a software-based dual-PC setup, I need to select the running **Steam Link application window** (or any specific window) to serve as the background layer behind the avatar.

### Proposed Functionality
* **Window Selector:** Add a "Screen Capture" or "Window Capture" option in the media source settings.
* **Specific App Target:** Allow selecting the specific window (e.g., "Steam Link") rather than just the full desktop, to avoid capturing UI elements or notifications from the Mac itself.
* **High Framerate Support:** Ensure the window capture can sustain 60fps, as this will carry the primary gameplay feed.

### Use Case Context
* **Streaming PC:** Mac Mini M4.
* **Gaming PC:** Windows (sending video via local network).
* **Workflow:**
1. Open Steam Link app on macOS.
2. Open Arkavo Creator.
3. Select Steam Link window as the "Background" or "Game Feed" source.
4. Overlay Avatar (using existing ARKit tracking).
5. Broadcast directly from Arkavo.

### Why this is important
This feature would allow Arkavo to support **Dual-PC streaming** without requiring users to purchase expensive hardware capture cards (Elgato/AverMedia), making the platform significantly more accessible for users migrating from OBS + VTube Studio setups.

Start by locating the media source settings and the existing camera-input or capture pipeline in the Swift macOS app. Determine how a specific application window could become a background video source while preserving the avatar overlay and broadcast path. Done means users can select a window such as Steam Link, avoid unrelated desktop UI, and receive a sustained 60fps feed.

Written by the indexing model from the issue text.