fix: remove invalid escape sequences in i18n placeholders (#800)

- Remove backslashes from source_patterns_placeholder in both en and zh
- Fix vue-i18n SyntaxError: 10 (invalid escape sequence)
- Change placeholder from 'e.g. @gmail\\.com$' to 'e.g. gmail.com'

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

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/views/admin/AccountSettings.vue

@@ -37,7 +37,7 @@ const { t } = useI18n({
             cancel: 'Cancel',
             config: 'Config',
             source_patterns: 'Source Address Regex (Optional)',
-            source_patterns_placeholder: 'e.g. @gmail\\.com$',
+            source_patterns_placeholder: 'e.g. gmail.com',
             source_match_mode: 'Match Mode',
             match_any: 'Any',
             match_all: 'All',
@@ -73,7 +73,7 @@ const { t } = useI18n({
             cancel: '取消',
             config: '配置',
             source_patterns: '来源地址正则（可选）',
-            source_patterns_placeholder: '例如: @gmail\\.com$',
+            source_patterns_placeholder: '例如: gmail.com',
             source_match_mode: '匹配模式',
             match_any: '任一',
             match_all: '全部',