fix(build): keep web apps on path routing through boot (no /#/, deep links survive) (#6639)

* fix(build): select web URL strategy before boot, not in prepareApp()

The build template called usePathUrlStrategy() from prepareApp(), which runs
after the boot UI mounts. By then Flutter's web routing had already initialized
under the default hash strategy, so path-routed web apps got /#/... URLs. Move
the strategy selection to the first statement in main(), before
FletDeepLinkingBootstrap.install() (which calls ensureInitialized) and any
runApp() — matching the before-boot ordering already used in client/lib/main.dart.

Necessary for path routing but not sufficient for cold-start deep links; see the
follow-up boot-overlay fix.

* fix(build): don't let the boot-screen MaterialApp report route '/' to the browser

The boot overlay wrapped the boot screen in MaterialApp(home: ...). A MaterialApp
with `home` builds its Navigator with reportsRouteUpdateToEngine: true, so on web
it pushes the home route '/' to the browser URL the moment it mounts — during
boot, before FletApp's real router reads the location. A cold-start deep link
like /gallery or /apps/<id> was therefore overwritten to '/'.

Verified with a headless-Chromium probe: pre-fix the URL reset /gallery -> /
~1.7s after 'Flutter app loaded' (before the Python worker even ran); post-fix
/gallery holds.

Render the boot screen via `builder:` instead of `home:`. With no
home/routes/onGenerateRoute, WidgetsApp creates no Navigator (per its own docs)
and reports nothing to the engine, so the deep link survives until the app's
router takes over. The overlay never navigates, so it needs no Navigator.

Regression from the 0.86 boot-screen rework (#6616); 0.85.3 rendered a bare
SizedBox during boot, which reported nothing.

Author: FeodorFitsner

sdk/python/templates/build/{{cookiecutter.out_dir}}/lib/main.dart

@@ -43,6 +43,20 @@ String appDir = "";
 Map<String, String> environmentVariables = Map.from(Platform.environment);
 
 void main(List<String> args) async {
+  // On web the URL strategy must be selected before ANYTHING reads the initial
+  // location: FletDeepLinkingBootstrap.install() (below) calls
+  // WidgetsFlutterBinding.ensureInitialized() and starts observing route pushes,
+  // and the first runApp() wires up the router. Whatever strategy is active at
+  // that point latches how the cold-start URL maps to a route — under Flutter's
+  // default hash strategy a hard refresh of `/apps/x` reads an empty fragment
+  // (route "/"), so the deep link is lost and the app rewrites the URL to "/".
+  // Applying the strategy that `flet build` baked into window.flet here, as the
+  // very first statement, keeps both the running URLs (no `/#/`) and cold-start
+  // deep links on the path strategy.
+  if (kIsWeb && getFletRouteUrlStrategy() == "path") {
+    usePathUrlStrategy();
+  }
+
   FletDeepLinkingBootstrap.install();
 
   _args = List<String>.from(args);
@@ -268,9 +282,16 @@ class _BootOverlayState extends State<_BootOverlay> {
         // resolveBootScreen is built once here (status changes update the
         // message via the screen's own ValueListenableBuilder), so the spinner
         // keeps animating across preparing → starting up.
+        // Use `builder:` rather than `home:` so this MaterialApp creates NO
+        // Navigator. A MaterialApp with `home` builds a Navigator with
+        // `reportsRouteUpdateToEngine: true`, which pushes the home route "/"
+        // to the browser URL on mount — clobbering a cold-start deep link
+        // (e.g. `/gallery`) before FletApp's real router reads it. The boot
+        // overlay never navigates, so it doesn't need a Navigator; this keeps
+        // theme/Directionality while leaving the URL untouched.
         child: MaterialApp(
           debugShowCheckedModeBanner: false,
-          home: resolveBootScreen(
+          builder: (context, _) => resolveBootScreen(
             name: bootScreenName,
             options: bootScreenOptions,
             extensions: extensions,
@@ -301,12 +322,9 @@ Future prepareApp() async {
   );
 
   if (kIsWeb) {
-    // web mode - connect via HTTP
+    // web mode - connect via HTTP. The URL strategy is applied in main() before
+    // runApp(); doing it here would be too late (see the note there).
     pageUrl = Uri.base.toString();
-    var routeUrlStrategy = getFletRouteUrlStrategy();
-    if (routeUrlStrategy == "path") {
-      usePathUrlStrategy();
-    }
     assetsDir = getAssetsDir();
   } else if (_args.isNotEmpty && isDesktopPlatform()) {
     // developer mode