Add lightweight Tauri shell, vendor offline libs, fix bugs and race conditions

- Add tauri/ subfolder: a Tauri-based desktop shell that loads the same
  index.html as the Electron app, producing a ~2MB installer (vs ~71MB for
  Electron) by using the OS WebView2 instead of bundled Chromium. Includes
  sync-frontend.js to keep tauri/dist/ in sync with the canonical index.html
  and vendor/ at build time (no UI duplication/drift).
- Vendor lucide, jsPDF, and jspdf-autotable locally under vendor/ instead of
  loading from CDNs, so the app works fully offline in both shells.
- Fix server.js path traversal vulnerability (sanitize/contain request paths).
- Fix exportPDF() using stale/edited piece data that could produce
  Infinity/NaN in generated PDFs; add a recalculate guard.
- Fix race conditions: overlapping copy/save-defaults button revert timers,
  and FileReader callbacks for logo/CSV import that could apply out of order.
- Minor cleanups: explicit array index parsing, hasResult() helper, consistent
  event binding for NMFC search results, cached DOM lookups.
- Document the dual-shell architecture and vendoring conventions in CLAUDE.md.

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

tauri/src-tauri/tauri.conf.json

@@ -0,0 +1,36 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "productName": "Freight Calculator Pro",
+  "version": "1.2.0",
+  "identifier": "com.freightcalc.pro",
+  "build": {
+    "frontendDist": "../dist"
+  },
+  "app": {
+    "windows": [
+      {
+        "title": "Freight Calculator Pro",
+        "width": 950,
+        "height": 950,
+        "minWidth": 800,
+        "minHeight": 700,
+        "resizable": true,
+        "fullscreen": false
+      }
+    ],
+    "security": {
+      "csp": null
+    }
+  },
+  "bundle": {
+    "active": true,
+    "targets": "all",
+    "icon": [
+      "icons/32x32.png",
+      "icons/128x128.png",
+      "icons/128x128@2x.png",
+      "icons/icon.icns",
+      "icons/icon.ico"
+    ]
+  }
+}