feat(sp6): /inbox route + sidebar nav link

src/App.tsx

@@ -11,6 +11,7 @@ import { ReconciliationPage } from "@/pages/Reconciliation";
 import { Acks } from "@/pages/Acks";
 import { Batches } from "@/pages/Batches";
 import { BatchDiff } from "@/pages/BatchDiff";
+import Inbox from "@/pages/Inbox";
 
 function NotFound() {
   return (
@@ -33,6 +34,7 @@ export default function App() {
           <Route path="activity" element={<ActivityLog />} />
           <Route path="upload" element={<Upload />} />
           <Route path="reconciliation" element={<ReconciliationPage />} />
+          <Route path="inbox" element={<Inbox />} />
           <Route path="acks" element={<Acks />} />
           <Route path="batches" element={<Batches />} />
           <Route path="batch-diff" element={<BatchDiff />} />

src/components/Sidebar.tsx

@@ -4,6 +4,7 @@ import {
   CheckCircle2,
   GitCompareArrows,
   GitMerge,
+  Inbox as InboxIcon,
   Layers,
   LayoutDashboard,
   Receipt,
@@ -102,6 +103,22 @@ export function Sidebar() {
               )}
             </NavLink>
           </li>
+          <li>
+            <NavLink
+              to="/inbox"
+              className={({ isActive }) =>
+                cn(
+                  "group relative flex items-center gap-3 rounded-md px-3 py-2 text-sm font-medium transition-colors",
+                  isActive
+                    ? "nav-active"
+                    : "text-muted-foreground hover:text-foreground hover:bg-muted/40"
+                )
+              }
+            >
+              <InboxIcon className="h-4 w-4" strokeWidth={1.5} />
+              <span>Inbox</span>
+            </NavLink>
+          </li>
           <li>
             <NavLink
               to="/acks"

src/pages/Inbox.tsx

@@ -0,0 +1,20 @@
+// ---------------------------------------------------------------------------
+// Inbox page (sub-project 6).
+//
+// Full-bleed dark working surface for the four-lane triage workflow.
+// Real implementation lands in T14-T17; this placeholder exists so the
+// /inbox route resolves during T13.
+// ---------------------------------------------------------------------------
+
+export default function Inbox() {
+  return (
+    <div
+      className="min-h-screen p-8"
+      style={{ background: "var(--tt-bg)", color: "var(--tt-ink)" }}
+    >
+      <p className="font-mono text-sm" style={{ color: "var(--tt-ink-dim)" }}>
+        Inbox — coming up in T14–T17
+      </p>
+    </div>
+  );
+}