feat(client): 购买卡年付明示省 2 个月（tab 标注+划线原价+省钱说明行）

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JJ1g8XV1YhhmHRzhwWEW7o

client/lib/screens/settings/purchase_card.dart

@@ -167,7 +167,7 @@ class _PurchaseCardState extends ConsumerState<PurchaseCard> {
           ),
           if (!_isPromo)
             DsSeg(
-              items: const ['年付', '月付'],
+              items: const ['年付 · 省 2 个月', '月付'],
               index: _cycleIdx,
               onChanged: (i) => setState(() => _cycleIdx = i),
             ),
@@ -252,6 +252,17 @@ class _PurchaseCardState extends ConsumerState<PurchaseCard> {
                       fontFamilyFallback: AppFonts.monoFallback)),
               const SizedBox(width: 8),
             ],
+            // 年付划线展示月付×12 原价，直观看到省 2 个月
+            if (!_isPromo && _cycleIdx == 0) ...[
+              Text('¥${_fmt(_group.monthly.price * 12)}',
+                  style: TextStyle(
+                      fontSize: AppDims.fsSm,
+                      color: t.faint,
+                      decoration: TextDecoration.lineThrough,
+                      fontFamily: AppFonts.mono,
+                      fontFamilyFallback: AppFonts.monoFallback)),
+              const SizedBox(width: 8),
+            ],
             Text('¥${_fmt(_plan.price)}',
                 style: TextStyle(
                     fontSize: 24,
@@ -265,6 +276,13 @@ class _PurchaseCardState extends ConsumerState<PurchaseCard> {
                     fontFamilyFallback: AppFonts.monoFallback)),
           ],
         ),
+        if (!_isPromo && _cycleIdx == 0) ...[
+          const SizedBox(height: 6),
+          Text(
+              '月付 ¥${_fmt(_group.monthly.price)} × 12 个月 = ¥${_fmt(_group.monthly.price * 12)}，'
+              '年付立省 ¥${_fmt(_group.monthly.price * 12 - _group.annual.price)}，相当于免费用 2 个月',
+              style: TextStyle(fontSize: AppDims.fsXs, color: t.success)),
+        ],
         const SizedBox(height: 6),
         Text('到期时间自动叠加，未到期续费不吃亏 · 支付宝支付',
             style: TextStyle(fontSize: AppDims.fsXs, color: t.faint)),