Implement UI enhancements, change password settings, theme toggle, and custom grid size slider

backend/handlers/auth.go

@@ -245,6 +245,46 @@ func (h *AuthHandler) Disable2FA(c *fiber.Ctx) error {
 	return c.JSON(fiber.Map{"message": "2FA disabled successfully"})
 }
 
+// ChangePassword allows an authenticated user to change their password.
+// PUT /api/auth/password
+func (h *AuthHandler) ChangePassword(c *fiber.Ctx) error {
+	username := c.Locals("username").(string)
+
+	var req struct {
+		OldPassword string `json:"old_password"`
+		NewPassword string `json:"new_password"`
+	}
+	if err := c.BodyParser(&req); err != nil {
+		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "invalid request body"})
+	}
+
+	if len(req.NewPassword) < 8 {
+		return c.Status(fiber.StatusBadRequest).JSON(fiber.Map{"error": "new password must be at least 8 characters"})
+	}
+
+	var currentHash string
+	if err := h.DB.QueryRow("SELECT password_hash FROM users WHERE username = ?", username).Scan(&currentHash); err != nil {
+		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "database error"})
+	}
+
+	if !verifyPassword(req.OldPassword, currentHash) {
+		h.Guard.RecordFailure(c.IP())
+		return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "incorrect current password"})
+	}
+
+	newHash, err := hashPassword(req.NewPassword)
+	if err != nil {
+		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to hash new password"})
+	}
+
+	if _, err := h.DB.Exec("UPDATE users SET password_hash = ? WHERE username = ?", newHash, username); err != nil {
+		return c.Status(fiber.StatusInternalServerError).JSON(fiber.Map{"error": "failed to update password"})
+	}
+
+	h.DB.AddAuditLog("password_changed", fmt.Sprintf("User '%s' changed their password", username), c.IP())
+	return c.JSON(fiber.Map{"message": "password changed successfully"})
+}
+
 // --- Password Hashing with Argon2 ---
 
 func hashPassword(password string) (string, error) {