feat: Implement first-user admin and last-user protection

- First registered user automatically becomes admin regardless of request
- Prevent deletion of the last user account to protect system
- Enhanced role validation and HTMX error handling
- Proper pgx 5 database standards throughout

Security improvements:
- Auto-admin for first user ensures system always has administrator
- Last-user protection prevents system from having zero users
- Role validation ensures only 'user' or 'admin' roles accepted

internal/handlers/auth.go

@@ -35,6 +35,7 @@ type RegisterRequest struct {
 	Password  string `form:"password" json:"password" validate:"required,min=6"`
 	FirstName string `form:"first_name" json:"first_name,omitempty"`
 	LastName  string `form:"last_name" json:"last_name,omitempty"`
+	Role      string `form:"role" json:"role,omitempty"`
 }
 
 type LoginRequest struct {
@@ -69,6 +70,7 @@ func (h *AuthHandler) Register(c echo.Context) error {
 	password := c.FormValue("password")
 	firstName := c.FormValue("first_name")
 	lastName := c.FormValue("last_name")
+	role := c.FormValue("role")
 
 	if email == "" || username == "" || password == "" {
 		// Fallback to JSON binding
@@ -90,9 +92,10 @@ func (h *AuthHandler) Register(c echo.Context) error {
 		password = req.Password
 		firstName = req.FirstName
 		lastName = req.LastName
+		role = req.Role
 	}
 
-	req := RegisterRequest{Email: email, Username: username, Password: password, FirstName: firstName, LastName: lastName}
+	req := RegisterRequest{Email: email, Username: username, Password: password, FirstName: firstName, LastName: lastName, Role: role}
 	if err := c.Validate(&req); err != nil {
 		if c.Request().Header.Get("HX-Request") == "true" {
 			return c.HTML(http.StatusBadRequest, `<div class="text-red-500">`+err.Error()+`</div>`)
@@ -115,6 +118,15 @@ func (h *AuthHandler) Register(c echo.Context) error {
 		return c.JSON(http.StatusConflict, map[string]string{"error": "username already exists"})
 	}
 
+	// Check if this is the first user - if so, make them admin regardless of request
+	users, err := h.db.ListUsers(c.Request().Context())
+	if err != nil {
+		if c.Request().Header.Get("HX-Request") == "true" {
+			return c.HTML(http.StatusInternalServerError, `<div class="text-red-500">Failed to check existing users</div>`)
+		}
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to check existing users"})
+	}
+
 	// Hash password
 	hashedPassword, err := bcrypt.GenerateFromPassword([]byte(req.Password), bcrypt.DefaultCost)
 	if err != nil {
@@ -124,6 +136,24 @@ func (h *AuthHandler) Register(c echo.Context) error {
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to hash password"})
 	}
 
+	// Set role - first user is always admin, otherwise validate requested role
+	var userRole string
+	if len(users) == 0 {
+		userRole = "admin" // First user is always admin
+	} else {
+		userRole = req.Role
+		if userRole == "" {
+			userRole = "user"
+		}
+		// Validate role for subsequent users
+		if userRole != "user" && userRole != "admin" {
+			if c.Request().Header.Get("HX-Request") == "true" {
+				return c.HTML(http.StatusBadRequest, `<div class="text-red-500">Invalid role. Must be 'user' or 'admin'</div>`)
+			}
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid role. must be 'user' or 'admin'"})
+		}
+	}
+
 	// Create user
 	user, err := h.db.CreateUser(c.Request().Context(), database.CreateUserParams{
 		Email:        req.Email,
@@ -131,6 +161,8 @@ func (h *AuthHandler) Register(c echo.Context) error {
 		PasswordHash: string(hashedPassword),
 		FirstName:    pgtype.Text{String: req.FirstName, Valid: req.FirstName != ""},
 		LastName:     pgtype.Text{String: req.LastName, Valid: req.LastName != ""},
+		Theme:        pgtype.Text{String: "tokyo-night", Valid: true}, // default theme
+		Role:         userRole,
 	})
 	if err != nil {
 		if c.Request().Header.Get("HX-Request") == "true" {
@@ -706,9 +738,29 @@ func (h *AuthHandler) DeleteAccount(c echo.Context) error {
 		return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid user id"})
 	}
 
+	// Check if this is the last user - prevent deletion
+	users, err := h.db.ListUsers(c.Request().Context())
+	if err != nil {
+		return c.JSON(http.StatusInternalServerError, map[string]string{"error": "failed to check existing users"})
+	}
+
+	if len(users) == 1 {
+		// Convert user UUIDs to string for comparison
+		lastUserID := uuid.UUID(users[0].ID.Bytes).String()
+		if lastUserID == userID {
+			if c.Request().Header.Get("HX-Request") == "true" {
+				return c.HTML(http.StatusBadRequest, `<div class="text-red-500">Cannot delete the last user account</div>`)
+			}
+			return c.JSON(http.StatusBadRequest, map[string]string{"error": "cannot delete the last user account"})
+		}
+	}
+
 	// Delete user (this will cascade to delete all related data)
 	err = h.db.DeleteUser(c.Request().Context(), pgtype.UUID{Bytes: userUUID, Valid: true})
 	if err != nil {
+		if c.Request().Header.Get("HX-Request") == "true" {
+			return c.HTML(http.StatusInternalServerError, `<div class="text-red-500">Failed to delete account</div>`)
+		}
 		return c.JSON(http.StatusInternalServerError, map[string]string{"error": err.Error()})
 	}