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>

tests/unit/triggers/trigger-detector.test.ts

@@ -242,7 +242,7 @@ describe('Trigger Detector', () => {
       expect(url).toContain('/form/');
     });
 
-    it('should build chat URL correctly', () => {
+    it('should build chat URL correctly with /chat suffix', () => {
       const baseUrl = 'https://n8n.example.com';
       const trigger = {
         type: 'chat' as const,
@@ -259,7 +259,8 @@ describe('Trigger Detector', () => {
 
       const url = buildTriggerUrl(baseUrl, trigger, 'production');
 
-      expect(url).toBe('https://n8n.example.com/webhook/ai-chat');
+      // Chat triggers use /webhook/<webhookId>/chat endpoint
+      expect(url).toBe('https://n8n.example.com/webhook/ai-chat/chat');
     });
   });