[web] Document viewport-fit=cover SafeArea support, URL percent-encoding preservation, and clean up web overview docs for Flutter 3.50

[kevmoo]

Page URL

• https://docs.flutter.dev/deployment/web
• https://docs.flutter.dev/release/breaking-changes
• https://docs.flutter.dev/platform-integration/web
• https://docs.flutter.dev/resources/architectural-overview

Describe the problem

Flutter 3.50 includes two web platform behavior changes (viewport-fit=cover SafeArea support and URL percent-encoding preservation) plus several outdated/broken web architecture statements on docs.flutter.dev (cc @parlough @sfshaza2):

1. viewport-fit=cover SafeArea support & layout breaking change: In Flutter 3.50 (flutter/flutter#191647), Flutter Web respects viewport-fit=cover in <meta name="viewport"> and populates FlutterView.viewPadding / MediaQuery.paddingOf / SafeArea from CSS env(safe-area-inset-*). Existing web apps that already had viewport-fit=cover in web/index.html (previously stripped by the engine) will now render edge-to-edge under notches and home indicators unless wrapped in SafeArea.
2. Browser history URL percent-encoding preservation: URLs passed to SystemNavigator.routeInformationUpdated are no longer percent-decoded before being written to browser history (flutter/flutter#189835), preserving %2F, %26, %23, and %20 across page refreshes.
3. Outdated 2019 HTML renderer prose & broken [hot reload behind a flag][] link: sites/docs/src/content/platform-integration/web/index.md#L30-L41 still describes the removed Dart-implemented HTML drawing layer, and sites/docs/src/content/resources/architectural-overview.md#L1084 contains an unresolved [hot reload behind a flag][] reference link.

Detailed page breakdown & affected targets (AI-assisted)

Expected fix

1. sites/docs/src/content/deployment/web.md & sites/docs/src/content/release/breaking-changes/

• Edge-to-edge mobile web rendering (viewport-fit=cover):
  • Document how adding viewport-fit=cover to <meta name="viewport"> in web/index.html (now included as a commented-out option in flutter create templates) enables full-bleed edge-to-edge rendering on iOS and Android browsers and populates FlutterView.viewPadding / SafeArea via CSS env(safe-area-inset-*) (flutter/flutter#191647).
  • Add a breaking-change / migration note for Flutter 3.50 warning that existing web apps that already had viewport-fit=cover in web/index.html will now render under device cutouts/notches unless UI content is wrapped in SafeArea (or viewport-fit=cover is removed).
• Browser history percent-encoding:
  • Note that BrowserHistory no longer calls Uri.decodeComponent on URLs passed to SystemNavigator.routeInformationUpdated (flutter/flutter#189835), preventing loss of percent-encoded delimiters (%2F, %26, %23, %20) in route paths and query parameters.

2. sites/docs/src/content/platform-integration/web/index.md (https://docs.flutter.dev/platform-integration/web)

• Rewrite ## How it works (L30-L41) to remove 2019 HTML-renderer prose ("We implemented the core drawing layer completely in Dart and used Dart's optimized JavaScript compiler...") and accurately describe compilation to JavaScript (dart2js / ddc) or WebAssembly (dart2wasm) paired with hardware-accelerated WebGL/WebGPU rendering via CanvasKit and Skwasm.

3. sites/docs/src/content/resources/architectural-overview.md (https://docs.flutter.dev/resources/architectural-overview)

• Fix L1083-L1084 (and therefore allows hot restart and [hot reload behind a flag][].): remove the broken [hot reload behind a flag][] reference link and state that dartdevc supports both hot reload and hot restart on the web by default (--web-experimental-hot-reload was removed in flutter/flutter#185994).
• Remove the unused [Web renderers]: /platform-integration/web/renderers link definitions in sites/docs/src/content/deployment/web.md#L196 and sites/docs/src/content/platform-integration/web/building.md#L197.