fix(desktop): preserve RGBA format for Tauri app icon (#6990)

# Description of Changes

- Replaced the Tauri application icon with an RGBA-formatted PNG.
- Added a root `.imgbotconfig` that excludes the Tauri icon from
automatic image optimization.
- Fixed the `desktop:test` compilation failure caused by
`tauri::generate_context!()` rejecting the previous non-RGBA icon.
- Prevented ImgBot from potentially converting the icon back to an
unsupported indexed PNG while optimizing its file size.
- Verified that the current icon uses PNG Color Type 6 (`Truecolour with
alpha`).

```sh

[desktop:test] error: proc macro panicked
[desktop:test]    --> src/lib.rs:202:12
[desktop:test]     |
[desktop:test] 202 |     .build(tauri::generate_context!())
[desktop:test]     |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
[desktop:test]     |
[desktop:test]     = help: message: icon /Users/runner/work/Stirling-PDF/Stirling-PDF/frontend/editor/src-tauri/icons/icon.png is not RGBA
[desktop:test] 
[desktop:test] error: could not compile `***-pdf` (lib) due to 1 previous error
[desktop:test] warning: build failed, waiting for other jobs to finish...
[desktop:test] error: could not compile `***-pdf` (lib test) due to 1 previous error
task: Failed to run task "desktop:test": exit status 101
Error: exit status 101

```

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

.imgbotconfig

@@ -0,0 +1,5 @@
+{
+  "ignoredFiles": [
+    "frontend/editor/src-tauri/icons/icon.png"
+  ]
+}