ui [nfc]: Define app-bar title text style all in one place

Much like we did in b75908b for
ContentTheme.textStylePlainParagraph, this is meant to collect all
the details that had been implicitly defining the app-bar title
style, from various sources:
- [_M3Typography.englishLike] or [_M3Typography.dense] (the latter
  for Japanese)
- [Typography.blackCupertino] or [Typography.blackMountainView]
  (depending on platform)
- `zulipTypography` in lib/widgets/text.dart
- [ColorScheme.onSurface]
, make them explicit, and make it clear that this doesn't inherit
from anything.

(Unlike in b75908b, this handles the wrinkle of giving the
appropriate textBaseline for our Japanese support. See a recent
commit that added localizedTextBaseline.)

If we were defining this within the scope of the global `ThemeData`,
rather than outside it (to add to it), we might instead have aliased
the details like so:

  final theme = Theme.of(context);
  theme.textTheme.titleLarge!
    .apply(color: theme.colorScheme.onSurface);

, which is much simpler; oh, well. That `textTheme` is the result of
complicated computations based on Typography in [Theme.of], and
[Theme.of] only works when you're below the right [Theme] in the
widget tree.

Next, we'll change the `color`, `fontSize`, and `height` to align
with the Figma. (We couldn't have just passed a `TextStyle` with
those three fields by themselves, even with `inherit: true`. If you
give AppBar a TextStyle for the title like this, it just takes it
literally, in place of the default style that applies when you don't
give the TextStyle.)

Author: chrisbobbe

lib/widgets/text.dart

@@ -12,12 +12,13 @@ import 'package:flutter/material.dart';
 /// an [AppBar]'s title, of an [ElevatedButton]'s label, and so on.
 ///
 /// As of writing, it turns out that these styles also flow naturally into
-/// most of our own widgets' text styles.
+/// many of our own widgets' text styles.
 /// We often see this in the child of a [Material], for example,
 /// since by default [Material] applies an [AnimatedDefaultTextStyle]
 /// with the [TextTheme.bodyMedium] that gets its value from here.
-/// A notable exception is the base style for message content.
-/// That style is self-contained and is not meant to inherit from this;
+/// There are exceptions, having `inherit: false`, that are self-contained
+/// and not meant to inherit from this.
+/// For example, the base style for message content;
 /// see [ContentTheme.textStylePlainParagraph].
 ///
 /// Applies [kDefaultFontFamily] and [kDefaultFontFamilyFallback],

lib/widgets/theme.dart

@@ -7,16 +7,30 @@ ThemeData zulipThemeData(BuildContext context) {
   return ThemeData(
     typography: zulipTypography(context),
     extensions: [ContentTheme(context)],
-    appBarTheme: const AppBarTheme(
+    appBarTheme: AppBarTheme(
       // Set these two fields to prevent a color change in [AppBar]s when
       // there is something scrolled under it. If an app bar hasn't been
       // given a backgroundColor directly or by theme, it uses
       // ColorScheme.surfaceContainer for the scrolled-under state and
       // ColorScheme.surface otherwise, and those are different colors.
       scrolledUnderElevation: 0,
-      backgroundColor: Color(0xfff5f5f5), // `bg-top-bar` in Figma
+      backgroundColor: const Color(0xfff5f5f5), // `bg-top-bar` in Figma
 
-      shape: Border(bottom: BorderSide(
+      titleTextStyle: TextStyle(
+        inherit: false,
+        color: const Color(0xff1a1b21), // colorScheme.onSurface
+        fontSize: 22.0,
+        letterSpacing: 0.0,
+        height: 1.27,
+        textBaseline: localizedTextBaseline(context),
+        leadingDistribution: TextLeadingDistribution.even,
+        decoration: TextDecoration.none,
+        fontFamily: kDefaultFontFamily,
+        fontFamilyFallback: defaultFontFamilyFallback,
+      )
+        .merge(weightVariableTextStyle(context)),
+
+      shape: const Border(bottom: BorderSide(
         color: Color(0x33000000), // `border-bar` in Figma
         strokeAlign: BorderSide.strokeAlignInside, // (default restated for explicitness)
       )),