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/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "freight-calculator-pro-tauri",
+  "version": "1.2.0",
+  "description": "Professional Freight Class Calculator (Tauri shell)",
+  "private": true,
+  "scripts": {
+    "sync": "node sync-frontend.js",
+    "dev": "npm run sync && tauri dev",
+    "build": "npm run sync && tauri build",
+    "serve": "node ../server.js"
+  },
+  "license": "ISC",
+  "devDependencies": {
+    "@tauri-apps/cli": "^2.11.2"
+  }
+}