feat(toolbox/native): add platform-specific toolbar thresholds for iOS and Android

[iospro]

Summary

• Added separate toolbar button thresholds for iOS (IOS_THRESHOLDS) and Android (ANDROID_THRESHOLDS) based on real device screen widths
• Middleware now selects the correct threshold set at runtime via Platform.OS
• Disabled auto-promotion of a single overflow menu button into the main toolbar
• NATIVE_THRESHOLDS kept as an alias to IOS_THRESHOLDS for Redux initial state compatibility

Changes

File	Details
toolbox/constants.ts	Added IOS_THRESHOLDS (428, 375) and ANDROID_THRESHOLDS (412, 360); NATIVE_THRESHOLDS aliased to IOS_THRESHOLDS
toolbox/middleware.native.ts	Added Platform.OS check to dispatch platform-specific thresholds on config load
toolbox/functions.native.ts	Disabled single-button overflow promotion logic

iOS breakpoints (points)

Width	Devices
500+	8 buttons (screensharing visible)
428	iPhone Pro Max
375	iPhone SE / 8
320	fallback

Android breakpoints (dp)

Width	Devices
500+	8 buttons (screensharing visible)
412	Pixel 7, Galaxy S24+
360	Samsung A-series, mid-range
320	fallback

Why disable single-button overflow promotion

The original logic automatically moved a button from the overflow menu into the main toolbar when only one button remained in overflow. This caused unpredictable toolbar layout: the "More" button would disappear and a random button would appear in its place depending on which features were enabled. With explicit per-platform thresholds the button sets are already carefully defined — auto-promotion contradicts that intention and produces inconsistent UX across devices and configurations.

[jitsi-jenkins]

Hi, thanks for your contribution!
If you haven't already done so, could you please make sure you sign our CLA (https://jitsi.org/icla for individuals and https://jitsi.org/ccla for corporations)? We would unfortunately be unable to merge your patch unless we have that piece :(.