add --sidebar-width variable and use it on issue sidebar

Author: silverwind

web_src/css/base.css

@@ -26,6 +26,7 @@
   --checkbox-size: 15px; /* height and width of checkbox and radio inputs */
   --page-spacing: 16px; /* space between page elements */
   --page-margin-x: 32px; /* minimum space on left and right side of page */
+  --sidebar-width: 296px;
 }
 
 @media (min-width: 768px) and (max-width: 1200px) {

web_src/css/repo.css

@@ -47,7 +47,7 @@
 
 .issue-content-right {
   margin: 0 !important;
-  width: 300px;
+  width: var(--sidebar-width);
 }
 
 .issue-sidebar-combo .ui.dropdown .item:not(.checked) .item-check-mark {

web_src/css/repo/home.css

@@ -1,6 +1,6 @@
 .repo-grid-filelist-sidebar {
   display: grid;
-  grid-template-columns: auto 296px;
+  grid-template-columns: auto var(--sidebar-width);
   grid-template-rows: auto auto 1fr;
   gap: var(--page-spacing);
 }