feat: import Chinese-localized Buzz source snapshot

Signed-off-by: cls_宁波本机 <908705107@qq.com>

mobile/lib/shared/theme/buzz_theme.dart

@@ -0,0 +1,103 @@
+import 'package:flutter/material.dart';
+
+import 'accent_colors.dart';
+import 'app_colors.dart';
+
+/// Name of the first-party Buzz theme. Buzz reuses the GitHub Light palette for
+/// every base color; the one thing that sets it apart is a branded gradient
+/// painted across the app's top section. Mirrors desktop, where the same
+/// gradient fills the sidebar canvas — see `data-buzz-sidebar` in
+/// `desktop/src/shared/styles/globals/theme.css`.
+const buzzThemeName = 'buzz';
+
+/// Name of the dark counterpart, which reuses the GitHub Dark palette and the
+/// dark-tuned gradient stops. Paired with [buzzThemeName] in `themePairs`, so
+/// the two behave as a single "Buzz" choice under System mode.
+const buzzDarkThemeName = 'buzz-dark';
+
+/// Whether [themeName] is either half of the Buzz pair. Both halves enable the
+/// gradient so System mode keeps it on across an OS light/dark switch.
+bool isBuzzTheme(String themeName) =>
+    themeName == buzzThemeName || themeName == buzzDarkThemeName;
+
+/// Whether the current widget tree is using the first-party Buzz treatment.
+bool isBuzzThemeContext(BuildContext context) =>
+    Theme.of(context).extension<AppColors>()?.topSectionGradient != null;
+
+/// Primary foreground for the mobile top navigation.
+///
+/// Every theme uses its own [ColorScheme.onSurface]. Buzz is the exception:
+/// its desktop-matching top gradient needs a neutral black or white foreground
+/// rather than the accent-derived color scheme foreground.
+Color navigationPrimaryForeground(BuildContext context) {
+  final scheme = Theme.of(context).colorScheme;
+  if (!isBuzzThemeContext(context)) return scheme.onSurface;
+  return scheme.brightness == Brightness.dark ? Colors.white : Colors.black;
+}
+
+/// Secondary label and placeholder foreground for the mobile top navigation.
+Color navigationSecondaryForeground(BuildContext context) {
+  final scheme = Theme.of(context).colorScheme;
+  if (!isBuzzThemeContext(context)) return scheme.onSurfaceVariant;
+  return navigationPrimaryForeground(context).withValues(alpha: 0.4);
+}
+
+/// Channel-section label and icon foreground for the mobile side navigation.
+///
+/// Section labels need more hierarchy than a placeholder. Buzz therefore uses
+/// a stronger neutral over its gradient, while all other themes preserve their
+/// established secondary foreground token.
+Color navigationSectionForeground(BuildContext context) {
+  final scheme = Theme.of(context).colorScheme;
+  if (!isBuzzThemeContext(context)) return scheme.onSurfaceVariant;
+  return navigationPrimaryForeground(context).withValues(alpha: 0.8);
+}
+
+/// Search-field surface for the mobile top navigation.
+Color navigationSearchSurface(BuildContext context) {
+  final scheme = Theme.of(context).colorScheme;
+  if (!isBuzzThemeContext(context)) return scheme.surfaceContainerHighest;
+  return navigationPrimaryForeground(context).withValues(alpha: 0.04);
+}
+
+/// A low-contrast navigation divider derived from the active theme foreground.
+Color navigationDivider(BuildContext context, double opacity) =>
+    navigationPrimaryForeground(context).withValues(alpha: opacity);
+
+/// Buzz renders with its fixed neutral foreground while preserving the stored
+/// wire accent so the user's choice returns on another theme.
+int effectiveAccentIndex(String themeName, String storedAccent) {
+  if (isBuzzTheme(themeName)) return neutralAccentIndex;
+  return accentIndexForWireValue(storedAccent) ?? defaultAccentIndex;
+}
+
+/// Gradient stops, matching desktop's `--buzz-gradient-*` custom properties.
+const _lightTop = Color(0xFFE6E6B6);
+const _lightBottom = Color(0xFFC4D0DA);
+const _darkTop = Color(0xFF4A4616);
+const _darkBottom = Color(0xFF0A1423);
+
+/// The Buzz gradient for the app's top section, or null when [themeName] is not
+/// a Buzz theme — in which case the section keeps its default frosted fill.
+///
+/// The stops are fully opaque: under Buzz the color replaces the frosted
+/// treatment rather than tinting it, matching desktop's solid sidebar canvas.
+///
+/// [brightness] comes from the applied color scheme rather than the theme name,
+/// so System mode picks the right stops as the OS switches.
+LinearGradient? buzzTopSectionGradient(
+  String themeName,
+  Brightness brightness,
+) {
+  if (!isBuzzTheme(themeName)) return null;
+
+  final isDark = brightness == Brightness.dark;
+  return LinearGradient(
+    begin: Alignment.topCenter,
+    end: Alignment.bottomCenter,
+    colors: [
+      isDark ? _darkTop : _lightTop,
+      isDark ? _darkBottom : _lightBottom,
+    ],
+  );
+}