refactor: use AppColors.primary as Material 3 theme seedColor

Replace hardcoded seedColor with AppColors.primary to ensure Material
widgets and custom components use the same accent color. This creates
a single source of truth for the app's primary color.

Benefits:
- Material widgets (AppBar, Dialogs, etc.) now match custom components
- Changing AppColors.primary updates the entire color scheme
- Eliminates visual inconsistencies between theme and custom colors

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Changed files
+2 -1
lib
+2 -1
lib/main.dart
···
import 'package:provider/provider.dart';
import 'config/oauth_config.dart';
+
import 'constants/app_colors.dart';
import 'models/post.dart';
import 'providers/auth_provider.dart';
import 'providers/comments_provider.dart';
···
title: 'Coves',
theme: ThemeData(
colorScheme: ColorScheme.fromSeed(
-
seedColor: const Color(0xFFFF6B35),
+
seedColor: AppColors.primary,
brightness: Brightness.dark,
),
useMaterial3: true,