Web UI: File browser does not validate PNG images for Application Covers

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### Is there an existing issue for this?

- [x] I have searched the existing issues

### Is your issue described in the documentation?

- [x] I have read the documentation

### Is your issue present in the latest beta/pre-release?

This issue is present in the latest pre-release

### Describe the Bug

The "Select File" file browser modal in the Web UI does not validate if the selected file is a PNG image when choosing an Application's Cover Image.

https://github.com/user-attachments/assets/816a6368-b331-41dd-b6f8-61dcc86688c2

**Reproduction Steps:**
1. Open the Sunshine Web UI and navigate to the **Applications** tab.
2. Click **Add New** or edit an existing application.
3. Scroll down to the **Image** input field and click the folder icon ("Select File").
4. In the file browser, select a non-PNG file (e.g., a `.md` file) and click **Select**.
5. The file browser accepts the file and updates the path, providing no feedback to the user that the file is invalid.

### Expected Behavior

The file browser modal should either filter out non-PNG files or display an error message if the user selects an unsupported file type.

### Additional Context

Because the Web UI lacks validation, users can mistakenly configure their apps with unsupported images. They won't realize it failed until they notice their game cover is missing on Moonlight, leading to a confusing user experience.

### Host Operating System

macOS

### Operating System Version

26.5.2

### Architecture

arm64/aarch64

### Package

macOS - Homebrew

### GPU Type

Apple Silicon

### GPU Model

n/a

### GPU Driver/Mesa Version

n/a

### Capture Method

AVCaptureScreen (macOS)

### Apps

```json
{
"apps": [
{
"image-path": "desktop.png",
"name": "Desktop"
},
{
"detached": [
"open steam://open/bigpicture"
],
"image-path": "steam.png",
"name": "Steam Big Picture",
"prep-cmd": [
{
"do": "",
"undo": "open steam://close/bigpicture"
}
]
}
],
"env": {
"PATH": "$(PATH):$(HOME)/.local/bin"
}
}
```

### Log output

```shell

```

### Online logs

_No response_

Start by locating the Web UI's file browser modal and the Application Image input, then reproduce the issue by selecting a non-PNG file such as a .md file. Done means unsupported files are filtered out or an error is shown when one is selected, while valid PNG selection continues to work.

Written by the indexing model from the issue text.