feat(deps): papaparse for expense CSV export

Replaces the hand-rolled `[fields].map(v => \`"\${v}"\`).join(',')`
pattern in expense-export.tsx with papaparse's Papa.unparse.

The previous version didn't handle:
- commas inside fields (would split rows mid-record)
- newlines inside fields (would terminate rows early)
- BOM for Excel-friendly encoding
- numeric/null normalization

Papa.unparse handles all of those + accepts a keyed-object row shape
that lets us define column order and get matching headers for free.

Verified: tsc clean, vitest 1315/1315.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -88,6 +88,7 @@
     "nodemailer": "^8.0.7",
     "openai": "^6.37.0",
     "p-limit": "^7.3.0",
+    "papaparse": "^5.5.3",
     "pdf-lib": "^1.17.1",
     "pdfkit": "^0.18.0",
     "pino": "^10.3.1",
@@ -130,6 +131,7 @@
     "@types/mailparser": "^3.4.6",
     "@types/node": "^20.19.0",
     "@types/nodemailer": "^8.0.0",
+    "@types/papaparse": "^5.5.2",
     "@types/react": "^19.2.14",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.1",