refactor: replace legacy flex utility classes and remove flex.scss

Replace all instances of prefixed flex utility classes (like flex-items-center, flex-justify-between, flex-full, flex-shrink-0) with their standard un-prefixed counterparts (items-center, justify-between, flex-1, shrink-0) across the codebase. Remove the deprecated src/static/scss/flex.scss partial file as it is no longer needed after these updates.

src/static/scss/flex.scss

@@ -1,40 +0,0 @@
-// 弹性布局
-.flex-items-center {
-  align-items: center;
-}
-
-.flex-justify-start {
-  justify-content: flex-start;
-}
-
-.flex-justify-end {
-  justify-content: flex-end;
-}
-
-.flex-justify-center {
-  justify-content: center;
-}
-
-.flex-justify-between {
-  justify-content: space-between;
-}
-
-.flex-col {
-  flex-direction: column;
-}
-
-.flex-wrap {
-  flex-wrap: wrap;
-}
-
-.flex-nowrap {
-  flex-wrap: nowrap;
-}
-
-.flex-full {
-  flex: 1;
-}
-
-.flex-shrink-0 {
-  flex-shrink: 0;
-}

src/static/scss/index.scss

@@ -1,6 +1,5 @@
 @import "./background.scss";
 @import "./border.scss";
-@import "./flex.scss";
 @import "./height.scss";
 @import "./font-family.scss";
 @import "./overflow.scss";