feat(pos): add 5 quick improvements — dark mode, email quotes, barcode scan, returns, offline catalog

1. Auto dark mode: detect system prefers-color-scheme, auto-switch industrial/modern theme
2. Email quotation endpoint: POST /quotations/:id/email sends HTML email via SMTP
3. Camera barcode scanner: BarcodeDetector API with getUserMedia overlay in catalog
4. Returns with warranty: POST /returns endpoint with stock restoration and sale status tracking
5. Partial offline catalog: cache top 500 parts in IndexedDB, search when offline

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pos/static/js/sync-engine.js

@@ -5,9 +5,10 @@
   'use strict';
 
   var DB_NAME    = 'nexus_pos_offline';
-  var DB_VERSION = 1;
-  var QUEUE_STORE     = 'sync_queue';
-  var INVENTORY_STORE = 'inventory_cache';
+  var DB_VERSION = 2;
+  var QUEUE_STORE       = 'sync_queue';
+  var INVENTORY_STORE   = 'inventory_cache';
+  var TOP_PARTS_STORE   = 'cached_parts';
 
   var db = null;
 
@@ -26,6 +27,11 @@
           inv.createIndex('sku', 'sku', { unique: false });
           inv.createIndex('name', 'name', { unique: false });
         }
+        if (!d.objectStoreNames.contains(TOP_PARTS_STORE)) {
+          var tp = d.createObjectStore(TOP_PARTS_STORE, { keyPath: 'part_number' });
+          tp.createIndex('name', 'name', { unique: false });
+          tp.createIndex('category', 'category', { unique: false });
+        }
       };
       req.onsuccess = function (e) {
         db = e.target.result;
@@ -156,6 +162,52 @@
     });
   }
 
+  // ─── Top parts cache (offline catalog) ────────────────────────
+  function cacheTopParts() {
+    return fetch('/pos/api/sync/top-parts').then(function (resp) {
+      if (!resp.ok) throw new Error('Sync top-parts failed: ' + resp.status);
+      return resp.json();
+    }).then(function (data) {
+      var parts = data.parts || [];
+      return openDB().then(function (d) {
+        return new Promise(function (resolve, reject) {
+          var tx = d.transaction(TOP_PARTS_STORE, 'readwrite');
+          var store = tx.objectStore(TOP_PARTS_STORE);
+          store.clear();
+          parts.forEach(function (p) { store.put(p); });
+          tx.oncomplete = function () {
+            console.log('[SyncEngine] Cached ' + parts.length + ' top parts');
+            resolve(parts.length);
+          };
+          tx.onerror = function () { reject(tx.error); };
+        });
+      });
+    });
+  }
+
+  function searchCachedParts(query) {
+    return openDB().then(function (d) {
+      return new Promise(function (resolve, reject) {
+        var tx = d.transaction(TOP_PARTS_STORE, 'readonly');
+        var req = tx.objectStore(TOP_PARTS_STORE).getAll();
+        req.onsuccess = function () {
+          var all = req.result;
+          if (!query) { resolve(all); return; }
+
+          var q = query.toLowerCase();
+          var filtered = all.filter(function (p) {
+            return (p.part_number && p.part_number.toLowerCase().indexOf(q) !== -1) ||
+                   (p.name && p.name.toLowerCase().indexOf(q) !== -1) ||
+                   (p.category && p.category.toLowerCase().indexOf(q) !== -1) ||
+                   (p.brand && p.brand.toLowerCase().indexOf(q) !== -1);
+          });
+          resolve(filtered);
+        };
+        req.onerror = function () { reject(req.error); };
+      });
+    });
+  }
+
   // ─── Connectivity helpers ─────────────────────────────────────
   function isOnline() {
     return navigator.onLine;
@@ -193,12 +245,14 @@
 
   // ─── Public API ───────────────────────────────────────────────
   window.SyncEngine = {
-    queueOperation:    queueOperation,
-    processQueue:      processQueue,
-    getQueueCount:     getQueueCount,
-    isOnline:          isOnline,
-    cacheInventory:    cacheInventory,
-    getCachedInventory: getCachedInventory
+    queueOperation:     queueOperation,
+    processQueue:       processQueue,
+    getQueueCount:      getQueueCount,
+    isOnline:           isOnline,
+    cacheInventory:     cacheInventory,
+    getCachedInventory: getCachedInventory,
+    cacheTopParts:      cacheTopParts,
+    searchCachedParts:  searchCachedParts
   };
 
 })();