feat(client): 授权 Tab 文案改为兑换券语义

激活卡片标题/说明/输入提示/按钮/成功提示对齐时长券：
"兑换激活码"、hint JIUKU-XXXX-XXXX、"时长叠加到当前授权之后"。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/lib/screens/settings/settings_screen.dart

@@ -455,11 +455,11 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.start,
           children: [
-            const Text('激活授权码',
+            const Text('兑换激活码',
                 style:
                     TextStyle(fontSize: 14, fontWeight: FontWeight.w600)),
             const SizedBox(height: 8),
-            const Text('输入从官方渠道获得的授权码以激活或续期',
+            const Text('输入购买或活动获得的激活码，时长将叠加到当前授权之后',
                 style: TextStyle(
                     fontSize: 13, color: AppTheme.textSecondary)),
             const SizedBox(height: 12),
@@ -469,7 +469,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                   child: TextField(
                     controller: _licenseKeyCtrl,
                     decoration: const InputDecoration(
-                      hintText: 'ey... 授权码',
+                      hintText: 'JIUKU-XXXX-XXXX',
                       isDense: true,
                       border: OutlineInputBorder(),
                       contentPadding:
@@ -487,7 +487,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
                             width: 16,
                             height: 16,
                             child: CircularProgressIndicator(strokeWidth: 2))
-                        : const Text('激活'),
+                        : const Text('兑换'),
                   ),
                 ],
               ],
@@ -514,7 +514,7 @@ class _SettingsScreenState extends ConsumerState<SettingsScreen> {
       if (mounted) {
         ScaffoldMessenger.of(context).showSnackBar(
           const SnackBar(
-              content: Text('授权激活成功'), backgroundColor: AppTheme.success),
+              content: Text('兑换成功，授权已更新'), backgroundColor: AppTheme.success),
         );
       }
     } catch (e) {