feat: complete custom app icon integration

[RONAK-AI647]

Overview

This PR completes the custom app icon settings feature, integrates a premium "Customize" tile directly into the icon selection grid, refactors data presets to be config-driven, and aligns the layout style to match the pre-existing Chat Background settings screen.

Changes

1. App Icon Customization Screen & Layout Alignment

• Refactored app/settings/customize-icon.tsx to use local component state.
• Aligned layout, spacing, section titles (e.g. PREVIEW), and divider lines (<Separator />) with app/settings/background.tsx.
• Used global tokens (CORNERS, HEIGHT) from theme/globals.ts to give the apply gradient button and standard "Reset to Default" button matching pill-shaped styles.

2. Live Icon Customization Picker & Modal

• Fixed initial HSV coordinate mapping inside components/icon-customizer/color-palette-modal.tsx so hue/saturation handles align correctly when the picker is opened.
• Fixed contrast issues on "Cancel" and "Use this color" buttons by moving label strings into children.
• Integrated the customizer sheet title with the global FONT_SIZE token.

3. Icon Selection Grid & Alternate App Icons

• Removed the redundant bottom button from app/settings/app-icon.tsx.
• Integrated a new dashed-border "Customize" card in the preset selection grid (components/app-icon-grid.tsx), styled with a brand blue-to-purple gradient.
• Wrapped native imports dynamically inside src/actions/app-icon.ts to prevent runtime crashes in Expo Go environments.

4. Dynamic Preset Mapping

• Replaced manually hardcoded color codes in src/data/icon-customizer-presets.ts with a map that dynamically derives preset swatches from scripts/icon-config.ts (ICON_VARIANTS), creating a single source of truth.

Testing & Verification

• Ran npm run test -> Passed all 601 tests across 31 suites.
• Ran npm run lint -> Passed cleanly with zero syntax or format warnings.

Closes #172

[RONAK-AI647]

App Color:

Customization

Color palette:

@mrspence DO test once regarding this feature!! Any if any color , grpahic or font is breaking the global theme/UI , do let me know !!
I have tried my best to maintain the global UI.