feat(ui,keys): granular masking, stats match, legacy format

- Make maskApiKey progressive for short keys to reduce exposure
- When aggregating usage, fall back to masked key to match stored stats
- Support legacy provider config `api-keys` by mapping to `api-key-entries`
- Add scrolling for provider lists >5 and reset styles when empty
- Improves privacy, fixes mismatched stats display, and preserves compatibility

styles.css

@@ -1446,12 +1446,15 @@ input:checked+.slider:before {
 
 /* 列表样式 */
 .key-list,
-.provider-list,
 .file-list {
     max-height: 400px;
     overflow-y: auto;
 }
 
+.provider-list {
+    /* 默认不限制高度，动态设置 */
+}
+
 .key-item,
 .provider-item,
 .file-item {