ui: Migrate to Material Design 3

This completes the checklist at zulip#225. I tested the screen-reader
interface change with VoiceOver on my iPhone.

Fixes: zulip#225

lib/widgets/app.dart

@@ -92,7 +92,6 @@ class ZulipApp extends StatelessWidget {
         if (Theme.of(context).platform == TargetPlatform.iOS) '.AppleSystemUIFont' else 'sans-serif',
         'Noto Color Emoji',
       ],
-      useMaterial3: false, // TODO(#225) fix things and switch to true
       // This applies Material 3's color system to produce a palette of
       // appropriately matching and contrasting colors for use in a UI.
       // The Zulip brand color is a starting point, but doesn't end up as

lib/widgets/lightbox.dart

@@ -133,6 +133,7 @@ class _LightboxPageState extends State<_LightboxPage> {
 
     final appBarBackgroundColor = Colors.grey.shade900.withOpacity(0.87);
     const appBarForegroundColor = Colors.white;
+    const appBarElevation = 0.0;
 
     PreferredSizeWidget? appBar;
     if (_headerFooterVisible) {
@@ -146,6 +147,7 @@ class _LightboxPageState extends State<_LightboxPage> {
         centerTitle: false,
         foregroundColor: appBarForegroundColor,
         backgroundColor: appBarBackgroundColor,
+        elevation: appBarElevation,
 
         // TODO(#41): Show message author's avatar
         title: RichText(
@@ -167,6 +169,7 @@ class _LightboxPageState extends State<_LightboxPage> {
     if (_headerFooterVisible) {
       bottomAppBar = BottomAppBar(
         color: appBarBackgroundColor,
+        elevation: appBarElevation,
         child: Row(children: [
           _CopyLinkButton(url: widget.src),
           // TODO(#43): Share image

lib/widgets/login.dart

@@ -392,19 +392,13 @@ class _PasswordLoginPageState extends State<PasswordLoginPage> {
       decoration: InputDecoration(
         labelText: zulipLocalizations.loginPasswordLabel,
         helperText: kLayoutPinningHelperText,
-        // TODO(material-3): Simplify away `Semantics` by using IconButton's
-        //   M3-only params `isSelected` / `selectedIcon`, after fixing
-        //   https://github.com/flutter/flutter/issues/127145 . (Also, the
-        //   `Semantics` seen here would misbehave in M3 for reasons
-        //   involving a `Semantics` with `container: true` in an underlying
-        //   [ButtonStyleButton].)
-        suffixIcon: Semantics(toggled: _obscurePassword,
-          child: IconButton(
-            tooltip: zulipLocalizations.loginHidePassword,
-            onPressed: _handlePasswordVisibilityPress,
-            icon: _obscurePassword
-              ? const Icon(Icons.visibility_off)
-              : const Icon(Icons.visibility)))));
+        suffixIcon: IconButton(
+          tooltip: zulipLocalizations.loginHidePassword,
+          onPressed: _handlePasswordVisibilityPress,
+          icon: const Icon(Icons.visibility),
+          isSelected: _obscurePassword,
+          selectedIcon: const Icon(Icons.visibility_off),
+        )));
 
     return Scaffold(
       appBar: AppBar(title: Text(zulipLocalizations.loginPageTitle),