feat: OS widgets foundation + price widget

[jvsena42]

Description

This PR adds the foundation for native iOS Home Screen widgets via WidgetKit, with the Bitcoin price widget as the first concrete example mirroring its in-app counterpart.

The foundation establishes the contract that future home-screen widgets will reuse:

• App Group group.bitkit is the only sync channel between the main app and the widget extension — no shared MMKV, redux-persist, or keychain.
• Per-widget options stores save/load configuration in App Group UserDefaults and trigger WidgetCenter.reloadTimelines whenever the in-app editor saves changes, so the home-screen widget reflects edits within ~1s.
• A stale-while-revalidate caching strategy: the main app writes the latest payload to the App Group, and the widget extension reads it synchronously from placeholder / getSnapshot, kicks off a fresh fetch in getTimeline, and falls back to cache (or a graceful error state) when offline.
• Shared models and styles (Inter Tight fonts, Colors, Fonts, TextStyle) are wired into both targets via PBXFileSystemSynchronizedBuildFileExceptionSet, keeping the extension binary minimal.
• Widgets honor widgetRenderingMode so they render correctly in both full-color and accented (Liquid Glass / StandBy) modes.

The price widget itself ships small, medium, and large families. Small shows the price and 24h change badge. Medium and large add a Charts-based sparkline that picks color from the change direction and respects rendering mode. Selected pairs, period, and the optional Bitfinex.com source line all flow from the in-app price widget editor.

Linked Issues/Tasks

Mirrors the foundation introduced on Android in bitkit-android#895.

Screenshot / Video

Simulator.Screen.Recording.-.iPhone.16.-.2026-05-05.at.14.09.49.mp4

Simulator.Screen.Recording.-.iPhone.16.-.2026-05-06.at.07.11.40.mp4

QA Notes

1. Add the widget from the gallery

• Long-press the home screen, tap +, search "Bitkit".
• Confirm "Bitcoin Price" appears with Small, Medium, and Large previews.
• Add Medium and confirm the price, change badge, and sparkline render with cached or live data.

2. Configuration flows through App Group

• Open Bitkit, edit the in-app price widget: change selectedPairs to include BTC/EUR, switch period to 1W, toggle showSource on.
• Within ~1s, confirm the home-screen widget reloads with the new pairs, the new period label, and the "Bitfinex.com" footer.

3. Rendering modes

• On iOS 18+, switch the home screen to the tinted/Liquid Glass style and confirm the widget renders with Color.clear background and tint-friendly text.

4. Unrelated regressions

• Open the in-app widgets editor and confirm Blocks, Facts, News, Weather, Calculator, Suggestions still render the same as on master.
• Run the unit tests in Xcode (Cmd+U) — no test changes are expected.

[jvsena42]

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[jvsena42]

Large widget removed in #542