Add checkout/custody feature for tracking items in personal possession

Items can now be checked out of their bin into "my custody" and later
checked back in or marked consumed. Adds checkout/checkin API endpoints,
a My Custody sidebar page, CheckoutFlow and CheckinFlow modals, and
updates ProductDetail, Inventory, ConsumeFlow, and MarkGoneFlow to
handle the new checked-out status. Bulk items prompt for remaining
weight on check-in.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/api.ts

@@ -119,6 +119,24 @@ export const api = {
       body: JSON.stringify(body),
     }),
 
+  checkoutInventoryItem: (
+    id: string,
+    body: { date: string },
+  ) =>
+    request<{ ok: true }>(`/inventory/${id}/checkout`, {
+      method: "POST",
+      body: JSON.stringify(body),
+    }),
+
+  checkinInventoryItem: (
+    id: string,
+    body: { date: string; binId: string; remainingWeight?: number },
+  ) =>
+    request<{ ok: true }>(`/inventory/${id}/checkin`, {
+      method: "POST",
+      body: JSON.stringify(body),
+    }),
+
   auditInventoryItem: (
     id: string,
     body: { date: string; mode: AuditMode; value: number; confirmedBy?: string },