refactor(frontend): move default content-area padding to page-shell.css

After page-shell.css landed, six of the seven panel pages still kept an
identical `.X-page .content-area { padding: 24px }` desktop rule, plus
three of them kept an identical `padding: 8px` mobile rule. Hoist both
defaults into page-shell.css under a single 6-page selector group and
delete the per-page copies.

What stays page-specific:
- IndexPage keeps its mobile override (padding 12px + padding-top: 64px
  for the fixed drawer handle clearance).
- ApiDocsPage keeps its tighter desktop padding (16px) and its own
  mobile padding-top: 56px.

Settings .ldap-no-inbounds also switches from #999 to
var(--ant-color-text-tertiary) for theme adaptation.

frontend/src/pages/clients/ClientsPage.css

@@ -1,13 +1,3 @@
-.clients-page .content-area {
-  padding: 24px;
-}
-
-@media (max-width: 768px) {
-  .clients-page .content-area {
-    padding: 8px;
-  }
-}
-
 .clients-page .ant-pagination-options-size-changer,
 .clients-page .ant-pagination-options-size-changer .ant-select-selector {
   min-width: 100px;

frontend/src/pages/inbounds/InboundsPage.css

@@ -1,13 +1,3 @@
-.inbounds-page .content-area {
-  padding: 24px;
-}
-
-@media (max-width: 768px) {
-  .inbounds-page .content-area {
-    padding: 8px;
-  }
-}
-
 .summary-card {
   padding: 16px;
 }

frontend/src/pages/index/IndexPage.css

@@ -1,7 +1,3 @@
-.index-page .content-area {
-  padding: 24px;
-}
-
 @media (max-width: 768px) {
   .index-page .content-area {
     padding: 12px;

frontend/src/pages/nodes/NodesPage.css

@@ -1,13 +1,3 @@
-.nodes-page .content-area {
-  padding: 24px;
-}
-
-@media (max-width: 768px) {
-  .nodes-page .content-area {
-    padding: 8px;
-  }
-}
-
 .nodes-page .summary-card {
   padding: 16px;
 }

frontend/src/pages/settings/SettingsPage.css

@@ -1,7 +1,3 @@
-.settings-page .content-area {
-  padding: 24px;
-}
-
 .settings-page .conf-alert {
   margin-bottom: 10px;
 }
@@ -52,6 +48,6 @@
 
 .ldap-no-inbounds {
   margin-top: 6px;
-  color: #999;
+  color: var(--ant-color-text-tertiary);
   font-size: 12px;
 }

frontend/src/pages/xray/XrayPage.css

@@ -1,7 +1,3 @@
-.xray-page .content-area {
-  padding: 24px;
-}
-
 .xray-page .header-row {
   display: flex;
   flex-wrap: wrap;

frontend/src/styles/page-shell.css

@@ -60,6 +60,23 @@
   background: transparent;
 }
 
+.index-page .content-area,
+.clients-page .content-area,
+.inbounds-page .content-area,
+.xray-page .content-area,
+.settings-page .content-area,
+.nodes-page .content-area {
+  padding: 24px;
+}
+
+@media (max-width: 768px) {
+  .clients-page .content-area,
+  .inbounds-page .content-area,
+  .nodes-page .content-area {
+    padding: 8px;
+  }
+}
+
 .loading-spacer {
   min-height: calc(100vh - 120px);
 }