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>

.claude/launch.json

@@ -0,0 +1,11 @@
+{
+  "version": "0.0.1",
+  "configurations": [
+    {
+      "name": "freight-calculator",
+      "runtimeExecutable": "node",
+      "runtimeArgs": ["server.js"],
+      "port": 3010
+    }
+  ]
+}