feat: add backgroundColor property to CourierInboxTheme

[mikemilla]

Summary

Adds a new optional backgroundColor property to CourierInboxTheme that allows users to customize the background color of the entire inbox view on both iOS and Android.

Resolves #32

Problem

Users cannot set the background color of the inbox. The issue reporter tried various approaches including components.messageList.container.backgroundColor but nothing works because the theme has no backgroundColor field.

Solution

Added backgroundColor?: string to the CourierInboxTheme TypeScript interface and bridged it to both native platforms:

• TypeScript (src/models/CourierInboxTheme.tsx): Added backgroundColor?: string to the CourierInboxTheme interface
• iOS bridge (ios/CourierInboxReactNativeManager.swift): Maps the backgroundColor string from the RN theme dictionary to the native CourierInboxTheme constructor
• Android bridge (android/src/main/java/com/courierreactnative/CourierInboxViewManager.kt): Maps the backgroundColor string from the RN theme ReadableMap to the native CourierInboxTheme constructor
• Docs (Docs/2_Inbox.md): Updated the styled inbox example to show the new property

Usage

const theme: CourierInboxTheme = {
  backgroundColor: '#0C111D',
  // ...other properties
};

<CourierInboxView
  theme={{ light: theme, dark: theme }}
/>

Dependencies

This PR requires corresponding changes in the native SDKs to add the backgroundColor property to CourierInboxTheme:

• courier-ios: Add backgroundColor: UIColor? to CourierInboxTheme and apply it to the inbox's stack view, scroll view, table view, and tab bar. Changes are prepared in branch cursor/add-inbox-background-color-d185 (pending push access).
• courier-android: Add @ColorInt backgroundColor: Int? to CourierInboxTheme and apply it to the root layout, tab layout, recycler view, and swipe refresh layout. Changes are prepared in branch cursor/add-inbox-background-color-d185 (pending push access).

The native SDK versions referenced by this package (Courier_iOS 5.8.0 and courier-android 5.3.0) will need to be updated to versions that include the backgroundColor property.

Files Changed

File	Change
src/models/CourierInboxTheme.tsx	Added backgroundColor?: string to interface
ios/CourierInboxReactNativeManager.swift	Bridge backgroundColor to native iOS theme
android/src/main/java/com/courierreactnative/CourierInboxViewManager.kt	Bridge backgroundColor to native Android theme
Docs/2_Inbox.md	Updated styled example with backgroundColor