feat: configurable widgets (iOS 17+)

[V3RON]

Summary

Widgets today show the same content for every user. The only way to personalise what a widget displays is to open the app and push new content via updateWidget(). This creates a frustrating gap: users who want a stock widget showing a specific ticker, or a news widget showing their preferred category, have no native way to express that preference.

Configurable widgets solve this. iOS 17 introduced AppIntentConfiguration, which surfaces a native "Edit Widget" sheet for any parameters you declare. This PR adds full support for that feature in Voltra — from plugin config through native Swift codegen to the React Native read API.

• parameters in plugin config — declare enum, bool, int, or double parameters; Voltra generates the required AppEnum, WidgetConfigurationIntent, and AppIntentConfiguration Swift types at prebuild time
• outdatedStatePath — pre-rendered holding state shown while the user has changed parameters but the app hasn't re-rendered yet (non-server widgets)
• getWidgetParameters(widgetId) — new synchronous JS API (iOS-only, @use-voltra/ios-client) that reads the current parameter values from the shared App Group
• Server-driven widgets — parameter values are automatically appended as query params on every server fetch; no app interaction needed
• VoltraConfigurableHomeWidgetProvider — reusable generic AppIntentTimelineProvider in the Swift package; generated code is a thin shell over it
• Placeholder fix — both VoltraHomeWidgetProvider and VoltraConfigurableHomeWidgetProvider now serve stale cached content from placeholder(in:) instead of showing a spinner
• Example app — new Greeting Widget with name/theme/emoji parameters and a Configurable Widget testing screen
• Docs — new "Configurable widgets" page, updated plugin configuration reference

Test plan

• Add greeting widget to home screen; confirm it renders initial state
• Long-press → Edit Widget; change name, theme, show emoji toggle
• Confirm widget shows outdated state immediately after saving
• Open app → navigate to Configurable Widget screen; confirm preview updates and widget re-renders
• Verify getWidgetParameters('greeting') returns the values selected in the edit sheet
• Confirm non-configurable widgets (weather, portfolio) are unaffected
• Confirm iOS <17 builds compile (static widgets unchanged, configurable structs are @available(iOS 17.0, *))