fix(welcome): 品牌名严格遵循 lockup 设计规范

按 genex-lockup.svg 规范修正品牌名渲染:
- "GEN" = 深色 #1A103A (solid)
- "EX" = 渐变 #D946EF → #E11D89 (ShaderMask)
- fontWeight: 800, letterSpacing: -0.3
之前错误地将整个"Genex"做成全渐变，不符合品牌 VI。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/genex-mobile/lib/features/auth/presentation/pages/welcome_page.dart

@@ -32,19 +32,33 @@ class WelcomePage extends StatelessWidget {
               ),
               const SizedBox(height: 12),
 
-              // Brand Name — 使用 logo 同色系渐变
+              // Brand Name — 遵循 lockup 设计: "GEN" 深色 + "EX" 渐变
+              Row(
+                mainAxisSize: MainAxisSize.min,
+                children: [
+                  Text(
+                    'GEN',
+                    style: AppTypography.displayLarge.copyWith(
+                      color: const Color(0xFF1A103A),
+                      fontWeight: FontWeight.w800,
+                      letterSpacing: -0.3,
+                    ),
+                  ),
                   ShaderMask(
                     shaderCallback: (bounds) => const LinearGradient(
-                  colors: [Color(0xFF9B5CF6), Color(0xFFD946EF)],
+                      colors: [Color(0xFFD946EF), Color(0xFFE11D89)],
                     ).createShader(bounds),
                     child: Text(
-                  'Genex',
+                      'EX',
                       style: AppTypography.displayLarge.copyWith(
-                    color: Colors.white, // ShaderMask 需要白色底
-                    letterSpacing: 2,
+                        color: Colors.white,
+                        fontWeight: FontWeight.w800,
+                        letterSpacing: -0.3,
                       ),
                     ),
                   ),
+                ],
+              ),
               const SizedBox(height: 6),
 
               // Slogan