…cons for improved dark/light mode handling (#13597)

## **Description**

**Issue:** Our app icons were stuck in dark mode and ignored light mode
settings in TestFlight builds (while working correctly locally). The
build process was stripping the light mode icons during compilation.

**Solution:** Implement transparent background app icons that leverage
iOS system defaults:
- Light mode: System automatically applies white background
- Dark mode: System automatically applies gradient black background
This matches our brand color schemes perfectly and simplifies our icon
management.

**Additional Changes:**
- Removed legacy `CFBundleIcons` dictionary from Info.plist
- Kept only the modern `CFBundleIconName` entry
- Simplified asset catalog configuration to use transparent icons

**Risk Assessment:**
- Primary risk is potential App Store rejection due to transparent app
icons
- However, many apps successfully use this approach
- Fallback backgrounds are consistent with our brand guidelines
- Simplifies our icon management and build process

### Testing:
- Verified icons display correctly in development
- Confirmed proper light/dark mode behavior locally

### Next Steps:
- Monitor App Store review process
- Have backup solution with explicit backgrounds if needed
- Document this approach for future reference

## **Related issues**

Fixes:
[#4226](MetaMask/MetaMask-planning#4226)

## **Manual testing steps**

1. Install a fresh app
2. Hold down on the home screen until the apps jiggle, indicating that
the icons can be moved/deleted/edited
3. Click on the "Edit" button at the top right or left depending on the
device, and click "Customize"
4. A bottom sheet should appear allowing you to toggle light, dark, and
tint
5. Confirm that those still work

## **Screenshots/Recordings**

### Release Build


https://github.com/user-attachments/assets/bf3365b6-ac27-4592-a48b-56ae9a28bd18

### **Before**

![Screenshot 2025-02-17 at 09 51
36](https://github.com/user-attachments/assets/cada73ae-4b0b-46ab-b8d8-b319bdfbe1ee)

### **After**


![after](https://github.com/user-attachments/assets/dee4e5c3-9d17-4ea7-a025-4851949d4fea)

## **Pre-merge author checklist**

- [x] I’ve followed [MetaMask Contributor
Docs](https://github.com/MetaMask/contributor-docs) and [MetaMask Mobile
Coding
Standards](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/CODING_GUIDELINES.md).
- [x] I've completed the PR template to the best of my ability
- [x] I’ve included tests if applicable
- [x] I’ve documented my code using [JSDoc](https://jsdoc.app/) format
if applicable
- [x] I’ve applied the right labels on the PR (see [labeling
guidelines](https://github.com/MetaMask/metamask-mobile/blob/main/.github/guidelines/LABELING_GUIDELINES.md)).
Not required for external contributors.

## **Pre-merge reviewer checklist**

- [x] I've manually tested the PR (e.g. pull and build branch, run the
app, test code being changed).
- [x] I confirm that this PR addresses all acceptance criteria described
in the ticket it closes and includes the necessary testing evidence such
as recordings and or screenshots.

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Co-authored-by: metamaskbot <metamaskbot@users.noreply.github.com>