fix(pos): keep edit-line modal from getting stuck on invalid qty; show clear 409 stock-shortage error instead of silently queuing offline

public/pos/pos-app.js

@@ -618,7 +618,20 @@ function editSave(){
   if (!line) { closeEditLine(); return; }
   const qty = Number((document.getElementById('editQty').value || '').replace(/[^0-9.]/g, ''));
   const price = Number((document.getElementById('editPrice').value || '').replace(/[^0-9.]/g, ''));
-  if (!Number.isFinite(qty) || qty <= 0) { return; }
+  const hint = document.getElementById('editHint');
+  // Bad qty — show an error in the hint, keep the modal open so the
+  // operator can fix it. (Previously the modal stayed open without
+  // any feedback, so a cleared/0 qty field left the screen grey and
+  // the operator had to long-press another line to escape.)
+  if (!Number.isFinite(qty) || qty <= 0) {
+    if (hint) {
+      hint.textContent = 'Enter a quantity of 1 or more.';
+      hint.style.color = '#c92a2a';
+    }
+    const qIn = document.getElementById('editQty');
+    if (qIn) { qIn.focus(); qIn.select(); }
+    return;
+  }
   // price === 0 is allowed (the operator can comp a line) but
   // negative isn't. If the input is empty, leave the price alone.
   const shelf = Number(line.p.selling_price ?? line.p.cost_price ?? 0) || 0;
@@ -627,6 +640,9 @@ function editSave(){
     else delete line.unitPriceOverride; // back to default
   }
   line.qty = Math.floor(qty);
+  // Reset the hint to its neutral default before closing so the next
+  // openEditLine() doesn't show a stale error from this round.
+  if (hint) { hint.textContent = 'Hold to edit again, or change a value and tap Save.'; hint.style.color = ''; }
   persistCart();
   render();
   renderCartDrawer();
@@ -831,6 +847,14 @@ async function confirmSale(){
     discount_input: state.discount && state.discount.kind !== 'none' ? state.discount.input : undefined,
   };
 
+  // Distinguish a permanent server-side rejection (insufficient
+  // stock, bad input) from a transient failure (server down, network
+  // blip). The former must NOT advance to the receipt page or get
+  // queued for offline retry — it's a real, human-fixable problem
+  // the operator needs to see. The latter is what the offline queue
+  // is for.
+  let serverError = null;
+
   try{
     const r = await window.posFetch('/checkout', {
       method:'POST', headers:{'Content-Type':'application/json'}, body: JSON.stringify(checkoutBody),
@@ -840,7 +864,18 @@ async function confirmSale(){
       sale.synced = true;
       sale.serverRef = j.sale && j.sale.id;
     } else {
-      throw new Error('server returned ' + r.status);
+      // Try to read the body so we can tell apart stock-shortage
+      // (409) from generic 4xx/5xx. The route returns JSON on every
+      // non-2xx path, so this should always parse.
+      let body = null;
+      try { body = await r.json(); } catch (_) { /* not json */ }
+      if (body && body.reason === 'insufficient_stock' && Array.isArray(body.shortages) && body.shortages.length) {
+        serverError = { kind: 'insufficient_stock', shortages: body.shortages };
+      } else {
+        const msg = body && body.message ? body.message : ('server returned ' + r.status);
+        serverError = { kind: 'other', message: msg };
+        throw new Error(msg);
+      }
     }
   }catch(e){
     console.warn('Sale queued for sync', e);
@@ -856,6 +891,31 @@ async function confirmSale(){
     }
   }
 
+  // Permanent server-side rejection — stop here, do NOT navigate to
+  // the receipt page, do NOT queue for retry. Show a clear error,
+  // re-enable the Complete button so the operator can fix the
+  // cart (remove a line, lower the qty) and try again. The cart
+  // stays open.
+  if (serverError) {
+    if (serverError.kind === 'insufficient_stock') {
+      const lines = serverError.shortages.map(s => {
+        const item = state.cart.get(s.stock_item_id);
+        const name = item && item.p
+          ? (item.p.description || item.p.sku || s.stock_item_id)
+          : s.stock_item_id;
+        return `${name}: only ${s.available} in stock (you asked for ${s.requested})`;
+      });
+      toast.error('Not enough stock — ' + lines.join('; '));
+    } else {
+      toast.error('Sale rejected: ' + (serverError.kind === 'other' ? serverError.message : 'Unknown error'));
+    }
+    if (btn) {
+      btn.disabled = false;
+      btn.innerHTML = '<svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M5 12l5 5 9-11"/></svg>Complete sale';
+    }
+    return;
+  }
+
   // ALWAYS navigate to receipt — never leave the user stuck on "Saving…"
   try{ sessionStorage.setItem('pos.lastSale', JSON.stringify(sale)); }catch(_){}
   state.cart.clear(); persistCart(); paintCart(); render();

public/pos/sw.js

@@ -1,5 +1,5 @@
 // Service worker — cache app shell, network-first for HTML + theme.js, cache-first for other static.
-const CACHE = "pos-v18";
+const CACHE = "pos-v19";
 const APP_SHELL = [
   './',
   './index.html',