Tint the kanban inbox column lighter than the status columns

New --inbox-bg token (light and dark) applied via .kanban__col--inbox so the
inbox reads as distinct from "To do" / "Doing" / "Done".

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/style.css

@@ -11,6 +11,8 @@
   --warn-bg: #fff4e5;
   --warn-border: #f0c48a;
   --error: #b3261e;
+  /* Kanban inbox column: a touch lighter than a status column (var(--bg)). */
+  --inbox-bg: #fcfcfd;
   color-scheme: light dark;
 }
 
@@ -24,6 +26,7 @@
     --warn-bg: #2e2415;
     --warn-border: #6b5220;
     --error: #f2b8b5;
+    --inbox-bg: #202027;
   }
 }
 
@@ -406,6 +409,10 @@ h1 {
   padding: 0.75rem;
 }
 
+.kanban__col--inbox {
+  background: var(--inbox-bg);
+}
+
 .kanban__head {
   display: flex;
   align-items: center;