fix: make nested SafeAreaProvider report its own frame and insets on web

[janicduplessis]

Summary

Fixes #713

On web, NativeSafeAreaProvider always reported window-level values: insets from the hidden env(safe-area-inset-*) measurement element and the frame as document.documentElement dimensions, no matter where the provider is rendered. Nested SafeAreaProviders therefore returned root window metrics from useSafeAreaFrame / useSafeAreaInsets, unlike iOS/Android where each provider reports its own view's frame and insets.

The provider now measures its wrapper element with getBoundingClientRect() (on react-native-web the View ref is the underlying DOM element):

• the frame is the element's rect in viewport coordinates, matching the native frame in window coordinates
• insets are the window env() insets clamped to the part of the safe area overlapping the element: max(0, windowInset - distanceFromThatEdge)
• a ResizeObserver re-measures when the element resizes, and a capture-phase window scroll listener re-measures when an ancestor scroll moves it
• Fallback: when the element can't be measured (SSR, no ResizeObserver support) it keeps the previous window-level behavior. The hidden-element env() transition mechanism is unchanged as the window-level inset source.

Known limitation: a layout shift that moves the provider without resizing it and without scrolling (e.g. a sibling collapsing above it) won't re-report until the next resize/scroll/env() change.

Adds jest-environment-jsdom as a devDependency for the new tests.

Test Plan

New jsdom tests in src/__tests__/NativeSafeAreaProviderWeb-test.tsx render the web provider with react-dom and stub getBoundingClientRect, getComputedStyle and ResizeObserver:

• a provider positioned away from the window edges reports its own rect as the frame and clamped (zero or partial) insets — these failed before this change with window-level values
• a full-viewport provider reports the same insets/frame as before (regression guard)
• metrics update when the observed element resizes, and when an ancestor scroll moves it without resizing it (also failed before, values stayed stale)
• without ResizeObserver it falls back to window metrics

Ran with yarn validate:jest (all suites pass), plus prettier/eslint/tsc.