fix: address code review issues for form trigger improvements

- Add form-data as direct dependency (was only in devDependencies)
- Add TypeScript interfaces (FormFieldValue, FormFieldOption) replacing any types
- Add FORM_FIELD_TYPES constants for type-safe switch statements
- Add isValidBase64() validation for file uploads with size limits
- Add MAX_FILE_SIZE_BYTES (10MB) constant with validation
- Update form-handler.test.ts for FormData instead of JSON
- Update trigger-detector.test.ts for chat URL /chat suffix

Conceived by Romuald Członkowski - www.aiadvisors.pl/en

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

package.json

@@ -146,6 +146,7 @@
     "dotenv": "^16.5.0",
     "express": "^5.1.0",
     "express-rate-limit": "^7.1.5",
+    "form-data": "^4.0.5",
     "lru-cache": "^11.2.1",
     "n8n": "^1.121.2",
     "n8n-core": "^1.120.1",