Add container weight tracking for weigh-based concentrate audits

Record the total weight of a jar (product + container) at acquisition so
audits can be done by simply re-weighing the sealed jar. The tare is
derived (containerWeight − productWeight), and the audit flow offers a
"Weigh container" toggle that auto-calculates remaining product from the
scale reading.

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

web/src/components/ProductDetail.tsx

@@ -67,6 +67,12 @@ export function ProductDetail({
       : []),
     ["Purchase date", fmt.date(item.purchaseDate, getStoredTimezone())],
     ["Bin", isCheckedOut ? "In your custody" : bin ? bin.name : <span style={{ color: "var(--ink-3)" }}>—</span>],
+    ...(item.containerWeight != null
+      ? [
+          ["Container weight", `${item.containerWeight.toFixed(2)}g`] as [string, React.ReactNode],
+          ["Tare (empty jar)", `${(item.containerWeight - item.weight).toFixed(2)}g`] as [string, React.ReactNode],
+        ]
+      : []),
     ["Audit cadence", `Every ${cfg?.cadenceDays ?? "—"} days · ${cfg?.auditMode ?? "—"}`],
     [
       "Cost per gram",
@@ -294,6 +300,18 @@ export function ProductDetail({
                   {cfg?.unit}). Re-audit to update.
                 </div>
               )}
+              {item.containerWeight != null && (
+                <div
+                  style={{
+                    fontSize: 11,
+                    color: "var(--ink-3)",
+                    marginTop: 6,
+                    fontStyle: "italic",
+                  }}
+                >
+                  Expected container total: {((item.containerWeight - item.weight) + est).toFixed(2)}g
+                </div>
+              )}
             </div>
           )}