revert(settings): drop redundant --el-main-padding override

The previous commit added `--el-main-padding: 0` next to `padding: 0`
on .settings-main and claimed the custom-property override "bypassed
specificity" — that is not how CSS variables work. The variable's
computed value is resolved on the same element it's being read from,
and the lookup still goes through the normal cascade. Since
.settings-main[data-v-xxx] and .el-main sit on the same DOM node,
both the direct `padding: 0` and the var override win or lose by the
exact same specificity comparison. The var line gives no extra
guarantee, so it's just noise.

src/views/settings/Settings.vue

@@ -175,12 +175,6 @@
   }
 
   .settings-main {
-    // Override the Element Plus CSS custom property too, not just the
-    // padding declaration — belt-and-braces in case any scoped-hash
-    // edge case lets the default `padding: var(--el-main-padding, 20px)`
-    // slip through. Most visible on About / Overview where there's no
-    // tool-card to visually absorb a stray 20px top pad.
-    --el-main-padding: 0;
     padding: 0;
   }
 </style>