Add optional email verification and registration control

Implemented email service infrastructure with SMTP support and optional email verification for self-hosted deployments. Registration flow now supports open/closed modes and hardcoded Apocapoc branding.

Key features:
- Email service with SMTP and template rendering
- Optional email verification (auto-verified without SMTP config)
- Registration modes: open/closed for access control
- Hardcoded Apocapoc branding (AppName, AppURL, DefaultFrom)
- Separate registration and login flows (registration no longer returns tokens)

internal/shared/constants/branding.go

@@ -0,0 +1,7 @@
+package constants
+
+const (
+	AppName     = "Apocapoc"
+	AppURL      = "https://apocapoc.app"
+	DefaultFrom = "noreply@apocapoc.app"
+)

internal/shared/errors/errors.go

@@ -3,9 +3,11 @@ package errors
 import "errors"
 
 var (
-	ErrNotFound           = errors.New("resource not found")
-	ErrAlreadyExists      = errors.New("resource already exists")
-	ErrInvalidInput       = errors.New("invalid input")
-	ErrUnauthorized       = errors.New("unauthorized")
-	ErrInvalidCredentials = errors.New("invalid credentials")
+	ErrNotFound            = errors.New("resource not found")
+	ErrAlreadyExists       = errors.New("resource already exists")
+	ErrInvalidInput        = errors.New("invalid input")
+	ErrUnauthorized        = errors.New("unauthorized")
+	ErrInvalidCredentials  = errors.New("invalid credentials")
+	ErrEmailNotVerified    = errors.New("email not verified")
+	ErrRegistrationClosed  = errors.New("registration is closed")
 )