Feat/notification preferences channel UI 264

[mijinummi]

Summary
Implemented: feat(frontend): integrate notification preference ui with channel and webhook orchestration #264

• Create NotificationPreferences component mapping to GET/PUT /notifications/preferences.
• Implement transactional multi-channel matrix binding for explicit email/webhook routes.
• Build developer webhook lifecycle components (POST /webhooks, DELETE /webhooks/:id).
• Add test dispatch payload button handlers providing interactive verification flows.

closes #264
closes #263