fix: register /api/dissectors route to fix 404 on settings page

[tito]

Summary

• The DissectorsListHandler was already defined in endpoint_rules.go but was never registered as a route in router.go
• This caused /api/dissectors to return 404, leaving the decoder dropdown on the settings page stuck on "loading..."
• Adds the missing GET /api/dissectors route

Test plan

• Open the settings page and verify the decoder dropdown loads with available dissectors
• Verify endpoint rule creation still works with the decoder selection