feat: server driven widgets

[DorianMazur]

Related #38 #52

Server Driven Widgets

Adds server-driven content updates for both iOS and Android home screen widgets, allowing widget UI to be rendered server-side and pushed to devices on a schedule.

What's included

Core infrastructure

• createWidgetUpdateHandler, node.js HTTP handler that accepts widget update requests and returns rendered payloads for both platforms
• Dual render paths, renderIos and renderAndroid
• Platform detection via family query param presence

iOS

• Keychain-based credential storage (VoltraKeychainHelper) shared between app and widget extension via App Groups
• VoltraWidgetServerFetcher for background timeline entry fetching
• Widget timeline provider integration with serverUpdate config

Android

• WorkManager-based periodic background fetching (VoltraWidgetUpdateWorker)
• Credential storage via Jetpack DataStore (VoltraWidgetCredentialStore)
• Widget receiver code generation with serverUpdate scheduling support

Demo

App

IOS Dynamic Widget	Android Dynamic Widget

Server

[vercel]

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