feat(appearance): add stroke and background color customization with app-adaptive and event-adaptive support

[anuragdeshpande]

Resolves #22 (#22)

Summary

This PR adds comprehensive stroke and background color customization for the Dynamic Island in both collapsed (Normal) and Expanded states, including App-Adaptive color extraction for notifications and full Event-Adaptive color support for system events (Wi-Fi, Charging, Lock/Unlock, Battery, etc.).

---

What Was Done

1. Stroke Outline Customization

• Added an outline/stroke toggle for the island with adjustable Stroke width (1–8 dp) and Stroke opacity (0–100%).
• Added Stroke colour selection with presets, custom color picker, Material You dynamic theme roles, and App Icon Adaptive Colour.
• For App Icon adaptive color, added Non-app event fallback strategies:
  • Adaptive: Uses the active event's primary accent or media art color.
  • Material You: Uses the system dynamic theme accent.
  • OLED Black: Uses pure black (#000000).

2. Collapsed & Expanded Background Customization

• Dedicated background fill configurations for Normal (collapsed) and Expanded states with smooth cross-fade animation during expansion/collapse.
• Three background modes:
  • Solid: Flat fill supporting fixed colors, Material You dynamic roles, OLED black, and App Icon adaptive color.
  • Gradient: Two-stop gradient with selectable direction (Horizontal, Vertical, Diagonal), start/end colors, and opacity.
  • App Fade: Directional fade (Left to right / Right to left) from the notifying app/event primary branding color into a base color (e.g. OLED black), with adjustable solid percentage, fade distance, and opacity.

3. Custom System Events Support (Wi-Fi, Charging, Lock/Unlock, etc.)

• Added unified primary color resolution for all events via IslandEvent.primaryColor().
• Primary color resolution follows a consistent hierarchy:
  1. Per-event user color override (colorOverride)
  2. Material You dynamic color when Dynamic color for all events (useThemeColor) is enabled
  3. Extracted app launcher icon color (appColor) for notifications
  4. Built-in event default accent (accent) for system events (Wi-Fi, USB, Charging, Battery, Headphones, etc.)
• Stroke and background configurations (including App Fade and App Icon adaptive stroke/background) are now applied to system events just like notifications, deriving the color from the event's primary color or dynamic theme.

4. Unit Testing & Verification

• Added JVM unit tests in CutoutColorsTest verifying color resolution for system events, color overrides, dynamic themes, app notifications, and empty/idle fallbacks.

---

Screenshots

Appearance & Stroke Settings	Background (Normal State)

Background (Expanded State)	Background App Fade Settings

---

Technical Details

• AppIconColorExtractor: Extracts dominant/vibrant branding color from the application's default launcher icon with fallback.
• AppearancePreferences: Stores stroke settings and serialized CutoutFill (Solid, Gradient, AppFade) and CutoutColor preferences via DataStore.
• CutoutColors: Color resolver supporting opacity, dynamic Material You roles, fallback strategies, and IslandEvent.primaryColor().
• DynamicIsland & IslandSurface: Renders stroke border and interpolates background brushes and base colors between normal and expanded states.
• BackgroundScreen & AppearanceScreen: Settings UI with preview integration, sliders, and fallback selection.

[EvanKoe]

It is a very cool feature, thanks for the PR. I am just wondering why the Appearance > Background color > App Fade is not integrated into the Appearance > Background color > Gradient, as both are gradients. Would that be possible for you?

Also, I love the stroke adaptive color feature, that is very nice, I might also use it myself! I still haven't figured out what does the "Non-app event fallback" do in Appearance > Stroke colour.

Also, you are trying to merge on main, not on dev. Maybe it's a settings on my end to force people to merge on dev, do you have any idea?

[EvanKoe]

The dev branch had a small refactor concerning colors, so it created conflicts. Now all the color logic moved to data/CutoutColor.kt instead of AppearanceSettings.kt. Can you rebase on dev and resolve the conflicts so I can merge your PR ? @anuragdeshpande

[anuragdeshpande]

Rebased onto dev and addressed your feedback:

1. Merged App Fade into Gradient: Consolidated background gradient logic so "App icon" can be picked as a color choice for gradient stops (with automatic migration for existing configurations), avoiding duplicate gradient management.
2. Customizable Text Colour: Added a text color picker with auto-contrast fallback so text remains legible across custom background fills/gradients.
3. Clarified Fallback Label & Description: Renamed "Non-app event fallback" to "Fallback colour" with an explanatory description ("Used when no app icon is available (such as system events or an idle island)").
4. Resolved Conflicts: Rebased cleanly on dev with the latest CutoutColor.kt architecture. All unit tests pass!

[EvanKoe]

Lgtm, might be a good idea (for later) to implement color animations (fade in-out) when adaptive color is on. Merging